The most common cause of system or security failures is lack of change control. Change control is a process that ensures that any changes to the system or the environment are authorized, documented, tested, and approved before implementation. Change control helps to prevent errors, conflicts, inconsistencies, and vulnerabilities that may arise from unauthorized or uncontrolled changes. Lack of change control can result in system instability, performance degradation, functionality loss, security breaches, or compliance violations.
Lack of system documentation, lack of physical security controls, and lack of logging and monitoring are also potential causes of system or security failures, but they are not as common or as critical as lack of change control. References: CISSP CBK Reference, 5th Edition, Chapter 3, page 145; CISSP All-in-One Exam Guide, 8th Edition, Chapter 3, page 113

The answer: Call back Systems; Callback systems provide access protection by calling back the number of a previously authorized location, but this control can be compromised by call forwarding. Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 35

A major objective with Configuration Management is stability. The changes to the system are controlled so that they don't lead to weaknesses or faults in th system.
The following answers are incorrect:
system maintenance. Is incorrect because it is not the best answer. Configuration
Management does control the changes to the system but it is not as important as the overall stability of the system.
system operations. Is incorrect because it is not the best answer, the overall stability of the system is much more important.
system tracking. Is incorrect because while tracking changes is important, it is not the best answer. The overall stability of the system is much more important.