When using Import Sets and Transform Maps to ingest data into the CMDB, it is critical that records are processed through the Identification and Reconciliation Engine (IRE) to prevent duplicates and enforce source precedence. In ServiceNow, this is achieved by invoking the IRE after the transform logic has completed.

The onAfter transform script is the correct place to call the IRE API. At this stage, the transformed data has already been mapped and prepared, allowing the IRE to correctly identify whether a CI already exists and reconcile updates according to defined rules.

The onBefore and onStart scripts execute too early---before data mapping is complete---making them unsuitable for IRE processing. The onComplete script runs after the entire import job finishes and is not intended for per-record CI identification and reconciliation.

Because Import Sets can bypass IRE if not configured correctly, using an onAfter script is a critical Data Foundations safeguard when this ingestion method is chosen.

Therefore, the correct answer is C -- onAfter.