Although all parts (postulates) of Dalton's atomic theory are important, which one of the postulates is crucial to explain the observations summarized by the Law of Definite Proportions?a. Atoms are very small.b. Atoms of the same elements have the same properties.c. Matter is composed of atoms. d. Atoms chemically combine with other atoms in fixed, whole-number ratios.

Law of Definite Proportions states that a given compound always contain its component elements in fixed ratio (by mass) and does not depend on its source and/or method of preparation.

If one is to explain the law of definite proportions properly, then Dalton's fourth atomic theory comes to mind which states that atoms chemically combine with other atoms in fixed, whole number ratios.

The law of definite proportions suggests that compounds always contain its component elements in fixed ratio no matter the source, for example, oxygen makes up 8/9 of the mass of the sample of pure water regardless of the source while hydrogen makes up the remaining 1/9 of the mass. This generally shows that when oxygen and hydrogen combine in the ratio 8:1, water molecule will be formed as suggested by the Dalton Atomic Theory.