I would go with #2. The Resource Management Plan would tell you how resources are acquired. But in this case the sponsor has assigned somebody to the team. The first step should be to determine if this change impacts the baselines / requires a change request.

Open to other opinions though.

Came across a question that says, a sponsor replaces a key resource with a new hire midway through a project and what should be the response from PM?
The below 2 choices came close,
1/ Refer resource management plan for next course of action
2/ Determine if resource change impacts the project scope, schedule, delivery timelines.

Which among the above options would you choose & why. Appreciate your thoughts.