Question

what is a mixture?
a mixture is composed of only one type of atom or one type of molecule
a mixture is composed of two or more pure substances that have been mixed together in a variable proportion

Explanation:

A mixture is defined as a combination of two or more pure substances where the substances are present in variable proportions. A pure - substance made of only one type of atom or molecule is not a mixture but an element or a pure compound respectively.

Answer:

A mixture is composed of two or more pure substances that have been mixed together in a variable proportion