What defines how well, or to what level a solution needs to perform?

Comprehensive and Detailed Explanation (paraphrased from AgilePM/Agile standards; exact long extracts cannot be provided):

In AgilePM (and broadly across agile practice), functional requirements describe what the solution should do—capabilities, behaviors, and services. By contrast, non-functional requirements (NFRs) describe how well the solution must perform those functions. NFRs encompass performance, reliability, security, usability, accessibility, supportability, maintainability, and other quality attributes that set objective thresholds (e.g., response times, availability targets, encryption standards). They guide architectural choices, testing strategies, and acceptance criteria, and they are essential to protecting quality during timeboxed delivery and MoSCoW prioritization. Testable acceptance checks can and should be derived from NFRs, but “testable requirements” is not the category name for defining performance levels—the recognized term is Non-Functional Requirements. Therefore, the correct option is B.