Unauthorized changes can be moved into production is the best concern that is addressed by securing production source libraries. Production source libraries contain the source code of programs that are used in the production environment. Securing production source libraries means implementing access controls, change management procedures, and audit trails to prevent unauthorized or improper changes to the source code that could affect the functionality, performance, or security of the production programs. The other options are less relevant concerns that may not be directly addressed by securing production source libraries, but rather by other controls such as program approval, version control, or change testing. References:
* CISA Review Manual (Digital Version), Chapter 4, Section 4.2.3.21
* CISA Review Questions, Answers & Explanations Database, Question ID 213

The first step in auditing a data communication system is to determine the business use and types of messages to be transmitted. This is because the auditor needs to understand the purpose, scope, and objectives of the data communication system, as well as the nature, volume, and sensitivity of the data being transmitted. This will help the auditor to identify the risks, controls, and audit criteria for the data communication system.
Traffic volumes and response-time criteria, physical security for network equipment, and the level of redundancy in the various communication paths are important aspects of a data communication system, but they are not the first step in auditing it. They depend on the business use and types of messages to be transmitted, and they may vary according to different scenarios and requirements. References: CISA Review Manual (Digital Version), [ISACA Auditing Standards]