According to the PMBOK Guide, 7th edition, section 2.3.2, "The project management plan is the primary source of information for how the project will be planned, executed, monitored, controlled, and closed. It integrates and consolidates all subsidiary plans and baselines, and other information necessary to manage the project." Therefore, the project manager should identify the goal of each work product, as this will help align the project activities with the project objectives and benefits. Clarifying project benefits with the sponsor (option A) is important, but not sufficient to ensure that the project management plan reflects those benefits.
Performing integration testing of each release (option C) and including the entire project team in testing (option D) are good practices for quality management, but not directly related to developing the project management plan. References:
* PMBOK Guide, 7th edition, section 2.3.2
* PMI-PBA Exam Practice Test and Study Guide, 2nd edition, page 179, question 12