Which validation does NIST require to confirm files and databases are complete and current to the last backup?

Answer: A. Data testing

SP 800-34r1 includes data testing in the validation of recovery, to verify that recovered data files or databases are complete and current to the last available backup, alongside functionality testing before returning to normal operations.

Source: NIST SP 800-34 Rev. 1 (NIST) — Sec. 4.4 Reconstitution Phase