B. Update the procurement management plan and negotiate with the contractor.
In this situation, the project manager should first update the procurement management plan to address the issue of missing materials and the contractor's offer. Then, the project manager should initiate negotiations with the contractor. This may involve discussing the cost of the missing materials, the timeline for delivery, and any other terms and conditions related to the procurement. Negotiating with the contractor allows the project manager to seek a more favorable agreement that aligns with the project's budget and schedule constraints. If necessary, the project manager can also involve relevant stakeholders, such as the project sponsor, to ensure that the negotiation process is conducted effectively.