Which of the following is an example of a functional requirement?

The chosen answer highlights a specific behavior or action that the system must enforce, which exemplifies a functional requirement. Functional requirements define what a system should do and include specific features and operations. In this case, the requirement prevents closing a batch when there are pending items, which directly ties into the system's functionality and its operational rules.

Functional requirements often address the interactions between the user and the system, the specific actions the system must perform, and the conditions under which those actions take place. By preventing the closing of a batch with pending items, the system ensures data integrity and operational consistency, which are critical aspects of its expected behavior.

The other options provide important operational guidelines or constraints but fall into different categories. For instance, a credit card number not being stored relates more to data handling and security requirements rather than direct functionality. The requirement for internal user experience to load in a particular timeframe touches on performance metrics, and daily data backup emphasizes reliability and availability rather than specific functional capabilities. While all these aspects are essential to the overall system design, they do not denote specific functionalities that the system must perform.