A “usability requirement” is

A usability requirement is a non-functional requirement that specifies the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use. It defines measurable criteria for the ease of use and user-friendliness of a system or component. These requirements are essential for guiding design and for establishing the basis against which the final product's usability will be formally tested and validated. They are distinct from functional requirements, which define what the system does.

A. This describes a detail of a test environment or context of use, not the definition of a usability requirement for the system itself.
B. This refers to the qualifications or prerequisites for the person conducting the test, not a quality attribute of the system under test.
C. This describes a requirement for the test process or methodology, not a requirement for the product's usability.

1. ISTQB® Certified Tester Foundation Level - Usability Testing Syllabus (Version 2018). Section 1.2, "What is Usability?", defines usability based on the ISO 9241-11 standard, stating it is the "extent to which a system, product or service can be used by specified users to achieve specified goals with effectiveness, efficiency and satisfaction in a specified context of use." A usability requirement is a formal statement of these desired characteristics.

2. ISO/IEC/IEEE 29148:2018, "Systems and software engineering — Life cycle processes — Requirements engineering." Section 6.3.3 discusses quality requirements, including usability. It specifies that such requirements should define the performance of the system from the user's perspective, directly supporting that a usability requirement is placed on the system.

3. Sommerville, I. (2011). Software Engineering (9th ed.). Pearson. In Chapter 4, "Requirements Engineering," Section 4.2.2, "Non-functional requirements," usability requirements are classified as product requirements that specify how well the system should perform regarding its ease of use for operators.