According to the PMBOK Guide, 6th edition, section 9.4.2.1, the project manager is responsible for managing the project team and ensuring that the team members are performing their assigned roles and responsibilities.
The project manager should also monitor and control the project work and address any issues or conflicts that arise among the team members or with other stakeholders. In this scenario, the project manager should talk with project B's leader to discuss the possible impacts of the frequent requests for assistance on both projects.
This will help to clarify the expectations and boundaries of the team members, as well as to identify and resolve any potential conflicts or resource constraints. The other options are either passive or aggressive, as they do not address the root cause of the problem or respect the authority and autonomy of the team members. References: PMBOK Guide, 6th edition, section 9.4.2.1, page 336.