RC5 is a symmetric encryption algorithm. It is a block cipher of variable block length, encrypts through integer addition, the application of a bitwise Exclusive OR (XOR), and variable rotations.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 4: Cryptography (page 153).

Section: Access Control
Explanation/Reference:
The CIA triad stands for Confidentiality, Integrity and Availability.

RC4 is a proprietary, variable-key-length stream cipher invented by Ron
Rivest for RSA Data Security, Inc. Skipjack, IDEA and Blowfish are examples of block
ciphers.
Source: SHIREY, Robert W., RFC2828: Internet Security Glossary, may 2000.

Explanation/Reference:
In response to an access-request from a client, a RADIUS server returns one of three authentication responses: access-accept, access-reject, or access-challenge, the latter being a request for additional authentication information such as a one-time password from a token or a callback identifier.
Source: TIPTON, Harold F. & KRAUSE, MICKI, Information Security Management Handbook, 4th Edition, Volume 2, 2001, CRC Press, NY, page 36.