Which of the following is a non-functional requirement?

The correct answer is associated with a non-functional requirement because it describes a system's performance characteristic, specifically its capacity to handle a certain number of concurrent users. Non-functional requirements are focused on how the system performs a task rather than the specific functions it performs. These requirements often concern aspects such as performance, scalability, security, usability, reliability, and efficiency.

In this case, stating that the system will allow up to 100 users to log in at the same time directly addresses the system's performance capability, which is a typical focus of non-functional requirements. This ensures that the system can handle a specific load, which is crucial for assessing its scalability and usability.

The other options primarily describe functional requirements—what the system should do. They outline specific actions related to using the system, such as enabling purchases or returns. Functional requirements are concerned with the behavior of the system in relation to user interactions or system operations.