Comprehensive and Detailed Explanation:

The most concerning issue in DLP implementations is when tuning has never been completed.

DLP solutions require fine-tuning to properly recognize sensitive data patterns and avoid false positives/false negatives.

If tuning is incomplete, the solution will either block legitimate business processes (too restrictive) or fail to detect actual leaks (too permissive).

Now let’s break down the options:

Option A: Server support limitations may be an issue, but DLP is primarily endpoint-focused here.

Option B: Implementing blocking mode without tuning can cause disruptions, but it is not as bad as never completing tuning.

Option D: Running in monitoring mode is acceptable in early stages of deployment (testing phase).

Therefore, never completing tuning (C) represents a fundamental control weakness and is the greatest concern.

???? ISACA Reference: CISA Review Manual 27th Edition, Domain 5, section on data leakage prevention and monitoring tools.