Explanation:

Let’s break it down:

- Aqua regia is famous for dissolving metals like gold and platinum–stuff that most acids can’t touch.

- The classic recipe: three volumes of concentrated hydrochloric acid and one volume of concentrated nitric acid.

- So, the correct choice is Option:2, Three.

- Why those proportions? Hydrochloric acid supplies chloride ions to help create chloroauric acid from gold, while nitric acid kicks off the oxidation process.

- Option:1, Two and Option:4, Four aren’t standard ratios used in any chemistry lab I know.

- Option:3, One–that’s just plain wrong, since you’d be way off the classic 3:1 mix.