Which of the following is a non-functional requirement?

Non-functional requirements specify criteria that judge the operation of a system, rather than specific behaviors or functions. These can cover various aspects such as performance, usability, reliability, and security, rather than detailing specific system functions.

The option stating that the system will allow up to 100 users to log in at the same time exemplifies a non-functional requirement related to performance and scalability. It sets a limit on the number of concurrent users, which directly relates to how well the system can handle load under specified conditions.

In contrast, the other choices outline functional requirements. Enabling users to buy books and allowing book returns are clearly specific functionalities that the system must provide. Similarly, ensuring the security of customer details, while it touches on an important aspect of system behavior, it is more about the function's implementation rather than detailing an operational attribute like performance or concurrency. Thus, the focus on the ability to manage multiple users simultaneously distinctly categorizes that choice as a non-functional requirement.