A usability test session is a controlled period during which a test participant performs tasks using the system under test while being observed by a moderator and sometimes additional stakeholders or observers. The goal is to understand how users interact with the interface and identify usability problems. Option A describes an individual task, not the whole session. Option C refers to a test plan or test script, and Option D describes a test technique rather than a usability session. Thus, option B provides the most accurate and comprehensive definition.