This question belongs to the Process domain because it concerns risk and issue management during execution.
PMI distinguishes between a risk, which is uncertain, and an issue, which is a problem that has already occurred and now requires action. Here, the vendor has already confirmed a 6-week delay, so the uncertain event has materialized. The project manager should therefore treat it as an issue and execute the planned response if one exists. PMI risk-response guidance notes that response plans should define what will be done, by whom, and under what trigger conditions.
Option B may eventually be one possible response, but the first step is to follow the established issue or risk response process. Option C delegates the problem away without first applying project controls. Option D is incorrect because this is no longer merely a risk needing identification; it is an active issue affecting the project. PMP-style thinking favors recognizing when a risk has occurred and moving promptly into the agreed response and issue-management process.