Which of the following is NOT a common operating system vulnerability?

The correct answer is C, as covert channels are considered a specific method of communication that can occur within a system, rather than a typical operating system vulnerability. Covert channels exploit the allowed mechanisms of a system to enable communication in a manner that does not comply with or is not intended by the system's security policies. While they can represent a security concern, they are less about systemic vulnerabilities within an operating system's architecture and more about the potential to misuse existing functionalities.

In contrast, poor configuration management, poor memory management, and insufficient system redundancy are all recognized vulnerabilities that can expose operating systems to various types of attacks and performance issues. Poor configuration management can lead to incorrect or insecure setups that attackers can exploit. Poor memory management can result in vulnerabilities like buffer overflows that may allow unauthorized code execution. Insufficient system redundancy can lead to a lack of reliable backup systems, which can be a point of failure in critical situations, leaving systems vulnerable to downtime and attacks.

Understanding these distinctions helps clarify how covert channels are not categorized in the same way as the other options, which are foundational issues that can arise in the development and management of operating systems.