C. Assess whether the vulnerability is within the organization's risk tolerance levels.

When a high-rated vulnerability is discovered just before go-live, the first step should be a risk-based decision—specifically, to:

Assess the risk in terms of impact and likelihood
Determine whether it falls within the organization's risk tolerance
Consider compensating controls, urgency of go-live, and contractual obligations

Only after this assessment can a decision be made to proceed, delay, or seek mitigation strategies.

Given the situation, the BEST way to proceed would be:

C. Assess whether the vulnerability is within the organization's risk tolerance levels.

Here's the rationale: