The basic unit of matter is the atom. It is the smallest particle of an element which can take part in chemical reactions and may or may not exist separately. It consists of sub-atomic particles, i.e., protons, neutrons and electrons.

The correct answer is option 'D', atom.

Explanation:
An atom is the smallest particle of an element that can take part in a chemical reaction. To understand why this is the case, let's break down the other options and understand their meanings:

a) Molecule: A molecule is formed when two or more atoms combine chemically. However, not all elements exist as molecules. Elements like hydrogen and oxygen exist as diatomic molecules (H2 and O2), while others like helium exist as single atoms (He). Therefore, a molecule is not always the smallest particle of an element that can take part in a chemical reaction.

b) Mixture: A mixture is a combination of two or more substances that are physically mixed together. In a mixture, the substances retain their individual properties and can be separated by physical means. While elements can certainly be part of a mixture, a mixture itself does not represent the smallest particle of an element that can take part in a chemical reaction.

c) Compound: A compound is a substance formed when two or more elements chemically combine in a fixed ratio. Compounds consist of molecules or ions that are made up of multiple atoms bonded together. While compounds are formed from elements, they are not the smallest particle of an element. Instead, compounds are the result of the combination of different elements.

d) Atom: An atom is the smallest unit of an element that retains the chemical properties of that element. Atoms consist of a nucleus, which contains protons and neutrons, surrounded by electrons. Atoms can combine with other atoms through chemical reactions to form molecules or compounds. When a chemical reaction occurs, it is the atoms that are rearranged or combined to form new substances.

In summary, the smallest particle of an element that can take part in a chemical reaction is an atom. Atoms combine to form molecules or compounds, but they themselves are the fundamental building blocks of matter.

The atom is the smallest particle that can take part in a chemical reaction. It is proved by Dalton in his atomic theory.