What is an example of a nonfunctional requirement?

The choice that identifies a nonfunctional requirement is one that addresses how a system operates rather than what the system does. Nonfunctional requirements pertain to the qualities and attributes of a system, such as performance, usability, reliability, and security.

In this context, the requirement that states the system must respond within 2 seconds is a clear example of a nonfunctional requirement because it specifies a performance metric. It does not describe a specific feature or function of the system, but rather the expected response time, which is a characteristic of the system's performance.

Other options reflect functional requirements or specifications. The indication that the system must support 100 users describes a capacity limit but is still related to a functional aspect of the system. The requirement to allow user registration specifies a function the system must perform. Similarly, the mention of a user-friendly interface pertains to usability but is not quantifiable in the way that response time is. Thus, while usability is important, it is expressed here in a more subjective manner compared to the precise performance requirement of a response time.