Unauthorized data modifications during conversion (D) are the most critical concern because they compromise data integrity, leading to inaccurate or corrupted information in the new system. Data migration should be performed with strict controls, including validation, reconciliation, and audit trails.
Other options:
Lack of online backups (A) is a risk but not as severe as compromised data integrity. Offline backups may still exist.
Undocumented change management (B) is a process deficiency but does not directly affect data accuracy.
Manual data conversion (C) increases risk but does not automatically indicate unauthorized changes.
Reference: ISACA CISA Review Manual, Information Systems Acquisition, Development, and Implementation