When a risk manager realizes that a project has unrealistic funding and low resources, the appropriate document to review is the Project Management Plan. The Project Management Plan includes detailed information about the project's budget, resources, and overall strategy. By reviewing this plan, the risk manager can identify any discrepancies between the planned and actual resources and funding, allowing them to assess the associated risks and take necessary corrective actions.
PMI's standards indicate that the Project Management Plan is the primary document that outlines how the project will be executed, monitored, and controlled, including how resources and budgets are allocated.