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Non-Functional Testing is focused on evaluating aspects of the software that pertain to how it performs under certain conditions, rather than what it does. It assesses characteristics such as performance, usability, reliability, and scalability, all of which are crucial from a user's perspective. This type of testing ensures that the system not only meets functional requirements but also behaves appropriately under various loads, user interactions, and environmental conditions.

While Functional Testing verifies that the system performs its intended functions correctly, it does not specifically address performance characteristics. Structural Testing, on the other hand, examines the internal workings of the system, and Integration Testing focuses on how different components interact with each other. Therefore, Non-Functional Testing is the most appropriate answer as it directly correlates with evaluating system performance from the user's viewpoint.