According to the PMBOK Guide, the project manager is responsible for ensuring that the project deliverables meet the acceptance criteria and obtaining formal acceptance of the deliverables from the customer or sponsor1. The project manager should also involve the relevant stakeholders in the validation and acceptance process to ensure their satisfaction and buy-in2. Therefore, the best option is to set up a meeting with stakeholders to validate and confirm acceptance of the deliverables. Option A is incorrect because the project sponsor is not the only stakeholder who needs to approve the deliverables. Option B is incorrect because updating the project management plan and marking the milestone as completed are actions that should be done after obtaining formal acceptance, not before. Option C is incorrect because asking the vendor to validate the agreement for any deviations is not a sufficient way to ensure the quality and compliance of the deliverables. References: 1: PMBOK Guide, 6th edition, section 4.6.3.2, p. 123 2: PMBOK Guide, 6th edition, section 13.4.2.2, p. 529