Step 1: Understanding Republic Act No. 6770.
Republic Act 6770 is the Ombudsman Act of 1989 (Philippines). It grants the Ombudsman investigatory, prosecutory, preventive, and public assistance powers.
Step 2: Analysis of powers.
- Prosecutory power: Granted.
- Preventive measures: Granted.
- Public assistant functions: Also granted.
Step 3: Conclusion.
Since all listed powers are provided, the answer is (D) None of the above.