The correct answer is (a). Aqua regia is a mixture of concentrated nitric acid and concentrated hydrochloric acid. It is a powerful oxidizing agent and can dissolve gold and platinum. It is used in gold refining and in etching metals.
Nitric acid is a strong acid that is corrosive to metals. It is also a powerful oxidizing agent. Hydrochloric acid is a strong acid that is also corrosive to metals. It is not as powerful an oxidizing agent as nitric acid, but it is still a strong enough oxidizing agent to dissolve gold and platinum.
Citric acid is a weak acid that is found in citrus fruits. It is not a strong enough oxidizing agent to dissolve gold or platinum. Benzoic acid is a weak acid that is found in many plants. It is not a strong enough oxidizing agent to dissolve gold or platinum.
Aqua regia is a dangerous substance and should be handled with care. It can cause severe burns to the skin and eyes. It can also be harmful if inhaled or ingested.