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A non-functional requirement is primarily concerned with how a system operates rather than what it does. This includes attributes such as performance, scalability, reliability, and security. Choosing the capacity to handle 100 concurrent users as a non-functional requirement is correct because it addresses the performance aspect of the system. Specifically, it defines the system's ability to support a certain number of simultaneous users, which directly relates to its scalability and user experience.

In contrast, the other options focus on functional aspects or specific operational processes. The ability to close opportunities with feedback pertains to a business process or functionality related to user interactions. The approval process for discounts relates to business logic and workflow, defining how a specific operational function must be executed. Lastly, the number of features in a product refers to the capabilities or functions of the system, which fall under functional requirements. Thus, the focus on scalability and performance in handling concurrent users distinctly identifies option B as a non-functional requirement.