According to the PMBOK Guide, a resource management plan is a component of the project management plan that describes how project resources are acquired, allocated, monitored, and controlled. It also includes the roles and responsibilities of project team members and other stakeholders, as well as the resource calendars and the process for managing changes to resource assignments. When a key subject matter expert (SME) leaves a multiyear project due to organizational downsizing, the project manager should refer to the resource management plan to determine how to replace the SME, how to transfer the SME's knowledge and expertise to the remaining team members, and how to communicate the change to the relevant stakeholders. The other options are not the best actions to take in this situation. Updating the stakeholder register is a possible consequence of the change, but not the first thing to do. Consulting the project sponsor may be helpful, but not as effective as following the established plan. Modifying the statement of work (SOW) is not necessary unless the scope of the project is affected by the loss of the SME. References: PMBOK Guide, 6th edition, pages
313-314, 318-319.