Which statement best describes a non-functional requirement?

A non-functional requirement refers to attributes related to the performance and quality of a system rather than specific behaviors or functions. The statement that the application must be available 24/7 highlights a key aspect of system availability, which is a critical non-functional requirement. This requirement assures users that the application is reliable and can be used without interruption at any time.

In contrast, the other statements primarily focus on functional capabilities or features of the system. For instance, the ability for users to create multiple accounts and the need for all users to have a unique identification are fundamental functionalities that dictate what the system should do. On the other hand, the statement regarding users receiving feedback within 5 seconds, while related to performance, specifically addresses response time rather than the broader operational availability of the system. Therefore, the first statement clearly embodies the characteristics of a non-functional requirement by emphasizing continuous system uptime.