Comprehensive and Detailed In-Depth Explanation:According to ISO 17021-1:2015, Clause 9.4.10 (Corrective Actions for Major Nonconformities):
* If a major nonconformity is not corrected within six months, the certification body must reject the certification request.
* Another Stage 2 audit (C) is not required unless the organization reapplies for certification.
* Restarting all audit activities (B) is unnecessary; instead, certification is denied.
Thus, A is the correct answer.