According to the PMBOK® Guide, specifically within the Project Schedule Management knowledge area, Define Activities is the process of identifying and documenting the specific actions to be performed to produce the project deliverables.

Key Purpose: The primary benefit of this process is that it decomposes work packages into schedule activities that provide a basis for estimating, scheduling, executing, monitoring, and controlling the project work.

Decomposition: This is the primary tool and technique used in this process. While the Create WBS process identifies the deliverables at the work package level, the Define Activities process takes those work packages and further breaks them down into the individual activities required to complete them.

Outputs: The main outputs of this process include the Activity List, Activity Attributes, and a Milestone List. These documents provide the necessary detail for the subsequent processes of sequencing and estimating durations.

Comparison with other options:

B. Sequence Activities: This is the process of identifying and documenting relationships among the project activities (e.g., determining which task must come first). It happens after the activities have been defined.

C. Define Scope: This is the process of developing a detailed description of the project and product. It focuses on what will be delivered (the boundaries of the project), whereas Define Activities focuses on the work (the actions) required to create those deliverables.

D. Control Schedule: This is a monitoring and controlling process. It is concerned with monitoring the status of the project to update the project schedule and managing changes to the schedule baseline, rather than the initial identification of activities.