Comprehensive and Detailed Explanation:

The most effective way to prevent unauthorized access and data theft is requiring multi-factor authentication (MFA), which adds an extra layer of security beyond just passwords.

Option A (Criminal background checks on all contractors) – Background checks help reduce risk but do not prevent credential misuse.

Option B (Reviewing access requests by the privacy office) – The privacy office may advise on best practices but is not responsible for granting or enforcing access controls.

Option C (Escalating access requests for approval by a data custodian) – While this improves oversight, it does not actively prevent credential misuse.

Option D (Requiring MFA) is the best solution because it ensures that even if a password is compromised, an additional authentication factor is required, reducing unauthorized access risks.