A project is delivering an integrated solution to an external client on a fixed-price contract.

According to the PMBOK® Guide, specifically within the Project Cost Management knowledge area, the project manager must refer to the established guidelines for managing and controlling costs, especially when a request for additional resources arises that was not originally budgeted.

Why Choice A is correct: The Cost Management Plan is the primary document that defines how the project costs will be planned, structured, and controlled. Crucially, it describes the level of authority for making financial decisions and the procedures for identifying and securing additional funding. In a fixed-price contract scenario, where the budget is rigid, the Cost Management Plan would specify the process for addressing budget overruns or requesting additional funds—including identifying who (e.g., the Program Manager, Sponsor, or Finance Department) is responsible for sourcing that budget.

Analysis of other options:

B (Business management plan): This is not a standard PMI document. While a " Business Case " or " Benefits Management Plan " exists, they focus on project justification and value realization, not the tactical responsibility of budget allocation for specific roles.

C (Stakeholder engagement plan): This plan outlines how to effectively engage stakeholders based on their needs and interests. While it helps identify who the stakeholders are, it does not define the financial procedures or budgetary responsibilities for resource acquisition.

D (Resource management plan): This plan identifies how to acquire, manage, and use physical and team resources. While it would help the technical lead define the roles of the two new developers, it typically defers to the Cost Management Plan to determine the financial " who " and " how " regarding the funding source for those resources.

In a complex structure involving a Technical PM, a Business Program Manager, and an External Client, the Cost Management Plan serves as the " source of truth " for financial governance and authority levels.