As per the law of definite proportion which is also called proust's law The composition of a substance depends on the total number of atoms and not on the composition of the mixture from which it was formed.
Like in the preparation of water 2 hydrogen molecule and one oxygen molecule reacts with each other to form 2 molecules of water. The reaction is 2[tex]H_{2}[/tex](g) + [tex]O_{2}[/tex](g) = 2[tex]H_{2}O[/tex](l). Here the origin of the components i.e. hydrogen and oxygen does not matter it depends upon the number of atoms present in the reactant and product. Thus among all the other interpretations- The composition of a substance depends on the total number of atoms and not on the composition of the mixture from which it was formed. This is correct.