According to the PMBOK Guide, a workstream is a logical grouping of project activities that produces a specific deliverable or sub-deliverable within a project. A workstream leader is the person who is responsible for planning, executing, monitoring, and controlling the workstream activities and deliverables. A functional manager is the person who has management authority over an organizational unit, such as a department, within a functional organization1.

In this scenario, the project manager faces a situation where one of the workstreams has missed its deadline multiple times, affecting the project schedule and performance. The project manager should speak to the workstream leader and the functional manager about this situation to ensure it does not happen again. The project manager should communicate with the workstream leader to understand the root causes of the delays, identify the risks and issues, and agree on the corrective and preventive actions. The project manager should also communicate with the functional manager to obtain the necessary resources, support, and guidance for the workstream leader and the workstream team. The project manager should facilitate the collaboration and coordination between the workstream leader and the functional manager to ensure the alignment of expectations, objectives, and deliverables1.

The other options are not as suitable as option C. Option A involves speaking to the workstream leader and the project sponsor. The project sponsor is the person or group who provides resources and support for the project and is accountable for enabling success1. However, the project sponsor is not directly involved in the management or execution of the workstream activities and deliverables. The project sponsor may not have the detailed knowledge or authority to address the specific issues or challenges faced by the workstream leader and the workstream team. Option B involves speaking to the workstream leader alone. This option may not be sufficient to resolve the situation, as the workstream leader may need the support and assistance of other managers or stakeholders to overcome the delays and improve the workstream performance. Option D involves speaking to the workstream leader and the project management office (PMO). The PMO is an organizational structure that standardizes the project-related governance processes and facilitates the sharing of resources, methodologies, tools, and techniques1. However, the PMO may not have the direct responsibility or authority over the workstream activities and deliverables. The PMO may not be able to provide the specific resources, support, or guidance needed by the workstream leader and the workstream team1.