Which sample of matter is a single substance? (1) air (3) hydrochloric acid (2) ammonia gas (4) salt water

Any substance that is made up of only one type of atom or molecule is said to be a pure single  substance whereas substance that is made up two or more different elements or molecule is said to be a mixture.

Air: Different types of gases are present in air that is nitrogen, water vapors, oxygen etc. So, air is a mixture.

Ammonia gas: It consists of only one molecule namely ammonia throughout the substance so it is a pure single substance.

Hydrochloric acid: It is made up by dissolving hydrochloric gas in water so it is a mixture.

Salt water: It is mixture as water and salt are being mixed which are two different substances.

Hence, ammonia gas is a single substance.

i think it's c. i'm not sure but i think so.