The correct answer is C. Apex unit test failures.

Because the unmanaged package was successfully installed in a Full Copy sandbox that matches production, issues like feature incompatibility, license mismatch, or object limits would normally have appeared during sandbox testing. The remaining Salesforce-specific production blocker is Apex test execution.

Salesforce deployment documentation states that if any Apex test fails, the deployment is rolled back. Salesforce Apex testing guidance also states that Apex unit tests must meet coverage requirements and complete successfully for deployment or packaging scenarios.

So the package failed in production because the required Apex tests did not complete successfully.

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