According to the PMBOK Guide, 7th edition, section 9.1.3, "The resource management plan is a component of the project management plan that provides guidance on how project resources should be categorized, allocated, managed, and released. It may be formal or informal, highly detailed or broadly framed, based on the needs of the project." Therefore, the project manager should have used the resource management plan to identify the impacts of the other projects on the availability and allocation of the shared resources, and to plan for contingencies or alternatives to avoid or mitigate the delay. Updating the risk management plan and registering lessons learned for the project (option A) may be done after the delay is resolved, but not before.
Reorganizing the team for the project based on the WBS (option B) may not be feasible or effective, as the shared resources may still have competing priorities or dependencies. Maintaining constant communication with the stakeholders of the other projects (option C) is a good practice, but not sufficient to prevent the delay from happening, as the project manager still needs to monitor and control the resource usage and performance. References:
* PMBOK Guide, 7th edition, section 9.1.3
* PMI-PBA Exam Practice Test and Study Guide, 2nd edition, page 181, question 16