Explanation:

• Option 1: Formic acid

- Not commonly used in gold and silver purification.

- Has some industrial uses but not related to precious metal refinement.

• Option 2: Nitric acid

- Main acid in purification of gold and silver.

- Used in the 'parting' process to separate silver from gold, as silver dissolves in nitric acid but gold does not.

- Plays a central role in refining precious metals.

• Option 3: Maleic acid

- Mainly used in organic synthesis, not in refining metals.

- Has no role in gold or silver purification.

• Option 4: Acetic acid

- Common in food industry (as vinegar) and for some cleaning, but not for precious metals purification.

- Not strong enough for gold or silver refinement.