Answer: Option (b) is the correct answer.
Step-by-step explanation:
A pure substance is defined as a substance that consists of fixed composition of atoms.
For example, NaCl is a pure substance as it contains same composition of sodium and chlorine atoms.
Whereas a substance which has varying composition of atoms is known as a mixture. For example, mixture of sand in water.
Thus, we can conclude that varying composition is not a property of a pure substance.