A usability test task is a clearly defined activity that a participant is asked to perform during a usability test session. These tasks are typically created by the test designer or moderator and are designed to reflect realistic user goals (e.g., "Find and buy a product"). The purpose is to observe the participant's interaction with the system and identify usability issues. Option A describes a usability test script, not a task. Option C defines usability evaluation in general, and D refers to a session, not a specific task. Therefore, B is correct.
References:
* ISO 25062:2006 - Common Industry Format (CIF) for Usability Test Reports
* Nielsen Norman Group: Writing Effective Usability Tasks
* Usability.gov: Usability Test Task Design
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