Question

it is composed of nitrogen. it is composed of oxygen. it is composed of carbon dioxide. it is composed of a mixture of gases.

Explanation:

The Earth's atmosphere (or a general gas mixture context) is made up of multiple gases like nitrogen, oxygen, carbon dioxide, argon, etc. So it's a mixture of gases. The other options are incorrect as they suggest it's composed of only one of those gases, but in reality, it's a combination.

Answer:

It is composed of a mixture of gases.