Which non-functional requirement would indicate the need for a system to remain operational under stress or high demand?

The need for a system to remain operational under stress or high demand is best captured by the concept of reliability. Reliability refers to the ability of a system to perform its required functions under stated conditions for a specified period of time. In the context of high demand or stress, reliability ensures that the system continues to operate correctly without failure.

A system that is reliable can handle increased workloads or stressful situations, thus preventing downtime or errors that can occur during peak usage periods. It is a critical non-functional requirement, particularly for applications that need to maintain consistent performance and user satisfaction, such as in e-commerce, financial services, or healthcare systems.

While other options such as performance, scalability, and availability are also important non-functional requirements, they focus on different aspects. Performance pertains to how quickly a system responds to requests, scalability refers to the ability to grow and manage increased loads by adding resources, and availability is about ensuring that the system is operational and accessible when needed. However, reliability specifically addresses the capability of the system to function correctly even when under high demand, making it the most suitable answer to the question.