The pure substance among the options provided is:
A. an atom
Explanation:
1. An atom is considered pure because it consists of only one type of element. It cannot be broken down further into simpler substances without losing its characteristic properties.
2. Air is a mixture of gases such as oxygen, nitrogen, carbon dioxide, and others. It is not a pure substance as it contains different elements in varying proportions.
3. A solution is a homogeneous mixture composed of a solute (substance being dissolved) and a solvent (the dissolving medium). Solutions are not considered pure substances due to their composition of different components.
Therefore, the correct answer to the question is A. an atom, as it represents a pure substance in its simplest form.
