According to the Project Management Professional (PMP) Reference Materials, the project manager should submit a change request to accelerate the project as requested by the customer when the key subcontractor asked for additional budget. This is because a change request is a formal proposal to modify any document, deliverable, or baseline of the project1. In this case, the change request would affect the project schedule, cost, and scope baselines, as well as the contract terms with the subcontractor. Therefore, the project manager should follow the change control process to document, analyze, approve, and implement the change request2.
The other options, A, B, and C, are not the first actions that the project manager should take. Asking upper management for more funds and updating the project budget may not be feasible or necessary without a proper justification and approval of the change request. Revising the project scope to cope with the budget changes may not satisfy the customer's needs or expectations, and may also require a change request. Updating the risk register and project log, and managing the budget closely are important activities, but they are not sufficient to address the change request and its implications. References: 1: A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Sixth Edition, Chapter 4.6.1, "Change Requests". 2: A Guide to the Project Management Body of Knowledge (PMBOK Guide) - Sixth Edition, Chapter 4.6.3, "Perform Integrated Change Control".