The correct answer is A.
A project manager must understand the legal implications of her actions. A contract is a legally binding document. Choice B involves just making notes. It does nothing to resolve the issue, so it cannot be best. There is no reason for the buyer to terminate (choice C), just because the seller does not want to continue. A signed contract is legally binding. If you selected choice D first, the seller could be taking such action as stopping work, removing resources, or otherwise negatively impacting the project while you are holding the meeting. It is best to first remind the seller that he is legally bound to continue. You could follow up that action with choice D.
Pass PMP in the First Attempt : PMP Strategy
Revise Procurement Management Concepts: Project Procurement Management
Quiz:
20 Questions Challenge
50 Questions Challenge