Answer: a. Atom
Explanation:
Atom is the fundamental and smallest unit of matter which contains electrons, protons and neutrons. It cannot be decomposed into simpler constituents using chemical reactions.
Mixture is a substance which has two or more components which do not combine chemically and do not have any fixed ratio in which they are present. Example: Air. It can be decomposed into simpler constituents using physical reactions.
Solution is a homogeneous mixture of a solute and a solvent. Example: salt in water. It can be decomposed into simpler constituents using physical reactions.
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass. Example: water [tex]H_2O[/tex]. It can be decomposed into simpler constituents using chemical reactions.
