Section: Risk, Response and Recovery
Explanation/Reference:
The MOST common threat that impacts a business's ability to function normally is power. Power interruption cause more business interruption than any other type of event.
The second most common threat is Water such as flood, water damage from broken pipe, leaky roof, etc...
Threats will be discovered while doing your Threats and Risk Assessments (TRA).
There are three elements of risks: threats, assets, and mitigating factors (countermeasures, safeguards, controls).
A threat is an event or situation that if it occured would affect your business and may even prevent it from functioning normally or in some case functioning at all. Evaluation of threats is done by looking at Likelihood and Impact of possible threat. Safeguards, countermeasures, and controls would be used to bring the threat level down to an acceptable level.
Other common events that can impact a company are:
Weather, cable cuts, fires, labor disputes, transportation mishaps, hardware failure, chemical spills, sabotage.
References:
The Official ISC2 Guide to the CISSP CBK, Second Edition, Page 275-276

From most effective (lowest CER) to least effective (highest CER) are: Iris scan, fingerprint, voice verification, keystroke dynamics. Reference : Shon Harris Aio v3 , Chapter-4 : Access Control , Page : 131 Also see: http://www.sans.org/reading_room/whitepapers/authentication/biometricselection-body-parts-online_139

Explanation/Reference:
Capacitance detectors monitor an electrical field surrounding the object being monitored. They are used for spot protection within a few inches of the object, rather than for overall room security monitoring used by wave detectors. Penetration of this field changes the electrical capacitance of the field enough to generate and alarm. Wave pattern motion detectors generate a frequency wave pattern and send an alarm if the pattern is disturbed as it is reflected back to its receiver. Field-powered devices are a type of personnel access control devices. Audio detectors simply monitor a room for any abnormal sound wave generation and trigger an alarm.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 10: Physical security (page 344).

Section: Network and Telecommunications
Explanation/Reference:
As per RFC, The DoD TCP/IP protocol model defines four layers, with the layers having names, not numbers, as follows:
Application (process-to-process) Layer:
This is the scope within which applications create user data and communicate this data to other processes or applications on another or the same host. The communications partners are often called peers. This is where the "higher level" protocols such as SMTP, FTP, SSH, HTTP, etc. operate.
Transport (host-to-host) Layer:
The Transport Layer constitutes the networking regime between two network hosts, either on the local network or on remote networks separated by routers. The Transport Layer provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. This is where flow-control, error- correction, and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections between internet hosts.
Internet (internetworking) Layer:
The Internet Layer has the task of exchanging datagrams across network boundaries. It is therefore also referred to as the layer that establishes internetworking; indeed, it defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines IP addresses. Its function in routing is to transport datagrams to the next IP router that has the connectivity to a network closer to the final data destination.
Link (network access) Layer:
This layer defines the networking methods with the scope of the local network link on which hosts communicate without intervening routers. This layer describes the protocols used to describe the local network topology and the interfaces needed to affect transmission of Internet Layer datagrams to next-neighbor hosts.
The DoD tcp/ip model DoD model Osi Model

Graphic above from: http://bit.kuas.edu.tw/
REALITY VERSUS THE STANDARD
In real life today, this is getting very confusing. Many books and references will not use exactly the same names as the initial RFC that was published. For example, the Link layer is often times called Network Access.
The same applies with Transport which is often times called Host-to-Host and vice versa.
The following answer is incorrect:
The session layer is defined within the OSI/ISO model but not within the DOD model. Being incorrect it made it the best answer according to the question. It does not belong to the DoD TCP/IP Model.
Reference(s) Used for this question:
http://www.freesoft.org/CIE/RFC/1122/
http://bit.kuas.edu.tw/~csshieh/teach/np/tcpip/

Section: Access Control
Explanation/Reference:
RFC 2828 (Internet Security Glossary) defines the Extensible Authentication Protocol as a framework that supports multiple, optional authentication mechanisms for PPP, including cleartext passwords, challenge- response, and arbitrary dialog sequences. It is intended for use primarily by a host or router that connects to a PPP network server via switched circuits or dial-up lines. The Remote Authentication Dial-In User Service (RADIUS) is defined as an Internet protocol for carrying dial-in user's authentication information and configuration information between a shared, centralized authentication server and a network access server that needs to authenticate the users of its network access ports. The other option is a distracter.
Source: SHIREY, Robert W., RFC2828: Internet Security Glossary, may 2000.