According to the PMBOK® Guide, specifically the Acquire Resources process, onboarding specific team members is a critical transition from planning to execution that impacts several management artifacts.

Resource Management Plan: While the plan initially outlines how resources will be acquired, it must be updated to reflect the actual resources assigned to the project. This includes their specific roles, responsibilities, and the timing of their involvement. Onboarding also triggers updates to the Project Team Assignments and Resource Calendars, which are sub-components or closely related to the Resource Management Plan.

Cost Baseline: In many organizations, resources are planned using " average " or " standard " rates. Once the project manager completes the actual onboarding, the specific costs (actual salaries, contractor rates, or specialized equipment costs) become known. If there is a significant difference between the estimated costs and the actual costs of the onboarded resources, the Cost Baseline must be updated to reflect the true financial commitment of the project.

The Transition: Onboarding is the point where " Generic Resource A " becomes " John Doe at $\$150$/hour. " This precision is what necessitates the baseline update.

Analysis of other options:

Option A: The Lessons Learned Register is typically updated after a process is completed to capture what went well or poorly. While you might update it eventually, it is a project document, not a component of the Project Management Plan.

Option C: The Procurement Management Plan governs the process of how you buy goods or services. Once resources are onboarded, you are executing that plan, not necessarily updating it (unless the procurement strategy itself changed).

Option D: Preassignment is a tool and technique (or an input) of the Acquire Resources process, not a component of the Project Management Plan that is updated after the activity. You cannot " update " a preassignment once the person is already onboarded.

Per PMI standards, when moving from resource planning to actual acquisition and onboarding, the project manager must ensure that the Resource Management Plan reflects the current team structure and the Cost Baseline remains accurate based on actual resource expenditures.