which postulate of daltons atomic theory can explain the law of definite proportion?

The law of definite proportion is also known as the law of constant proportion. This law states that in a compound, the elements are always present in definite proportions by mass’. 

The postulates of Dalton’s atomic theory are as follows.

1. All matter is made up of very tiny particles. These particles are called atoms.
2. An atom cannot be divided further i.e., atoms are indivisible.
3. Atoms can neither be created nor destroyed in a chemical reaction.
4. All atoms of an element are identical in all respects, including the mass, chemical properties, etc.
5. Atoms of different elements have different masses and chemical properties.
6. Atoms of different elements combine in small whole number ratios to form compounds.
7. In a given compound, the relative number and types of atoms are constant.

So the last postulate of Dalton's theory, which states that - 'In a given compound, the relative number and types of atoms are constant' explains the law of definite proportion.