The PM is building a Work Breakdown Structure (WBS). A WBS is the structured decomposition of the project's deliverables and scope into progressively smaller pieces until you reach the lowest practical level of detail (often called work packages), which can then be scheduled, estimated, assigned, and tracked. In the CompTIA Project+ objectives, the WBS is explicitly listed as a planning artifact used to break down the work during initiation/planning activities.
The other options don't fit the "smallest activity components" description. A milestone chart highlights key checkpoints/dates, not the granular decomposition of work. A task board visualizes workflow status (e.g., to- do/in-progress/done) but doesn't inherently define the decomposition structure. A detailed project plan is broader and would include schedule, resources, budget, and governance-often informed by the WBS rather than being the decomposition itself.
By creating the smallest components list, the PM is effectively decomposing scope into manageable units, which is precisely the purpose of a WBS in Project+ planning and control.