Which of the following is considered a nonfunctional requirement?

A nonfunctional requirement defines how a system performs a specific function rather than detailing what the system does. Usability is a key aspect of nonfunctional requirements because it focuses on the user experience, including how easy and efficient it is for users to interact with the system. This includes considerations such as learnability, accessibility, and overall user satisfaction.

In contrast, functional requirements would specify particular features or functions of the system, such as what the product should do or the tasks it must accomplish. For instance, implementation refers to the technical specifics of how a feature is developed, while features describe the distinct capabilities of the system. Functions are the tasks or behaviors that the system must carry out. Thus, usability stands out as a crucial aspect of the user experience that influences the effectiveness of how well those functions are realized, defining it as a nonfunctional requirement.