Which of the following is not a focus area for hardening an operating system?

The focus on hardening an operating system includes various practices that enhance security by reducing the attack surface and minimizing vulnerabilities. Installing necessary updates is actually a fundamental aspect of maintaining an operating system's security rather than a hardening practice. Regular updates address known vulnerabilities and exploits, ensuring that the system is protected against the latest threats.

In contrast, removing unnecessary services and libraries is a specific hardening step aimed at reducing potential attack vectors by eliminating components that are not needed for the system's function. Limiting administrator access is another critical step in hardening that ensures that only authorized personnel have higher privileges, thereby reducing the risk of accidental or malicious changes. Implementing a personal firewall adds an additional protective layer, helping to monitor and filter incoming and outgoing traffic, which is also part of a robust hardening strategy.

Thus, focusing on opportunities to patch and update is integral to maintaining security, but it does not constitute a specific hardening measure on its own, making it the correct choice in this context.