The correct answer is "A group of independent servers that are managed as a single system". A
server cluster is a group of servers that appears to be a single server to the user.
Answer "A primary server that mirrors its data to a secondary server" refers to redundant servers.

System audits and monitoring are the two methods organizations use to maintain operational assurance. Although the terms are used loosely within the computer security community, a system audit is a one-time or periodic event to evaluate security, whereas monitoring refers to an ongoing activity that examines either the system or the users. In general, the more real-time an activity is, the more it falls into the category of monitoring. Source: NIST Special Publication 80014, Generally Accepted Principles and Practices for Securing Information Technology Systems.

The correct answer is "Configuration review", a distracter.
*Answer configuration control involves controlling changes to configuration items and issuingversions of configuration items from the software library.
*Configuration status accountingis the processing of changes.
* Configuration audit is the process of controlling the quality of configuration management procedures.

Other elements are included in a PKI.
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 165).