According to dalton's atomic theory:

a. an atom cannot be subdivided into smaller particles.
b. an atom can be subdivided into smaller particles.
c. the nucleus is made up of protons and neutrons.
d. electrons are lighter than protons.

An atom cannot be subdivided into smaller particles. (Option A).

Explanation:

According to Dalton's atomic theory: All matter is composed of tiny particles called as atoms. Some postulates of this theory are given below:

1) An atom cannot be subdivided into smaller particles it means they are indivisible.

2) Different element atoms have different masses, and chemical properties.

3) All atoms which are present in an element are identical in all respects such as mass, chemical properties, etc.

4) Atom cannot create or destroyed in a closed system.

5) The types of atoms, and relative number are constant in a given compound.