In any chemical compound, the elements are always combined in the same proportion by a. charge b. mass c. volume d. density

Answer is: B. mass.

Proust's law or law of constant composition said that a given chemical compound always contains its component elements in fixed ratio and does not depend on method of preparation.

Law of multiple proportions or Dalton's Law said that the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.  

For example, nitrogen(I) oxide N₂O; m(N) : m(O) = 2·14 : 16 = 7 : 4.

i just learnt this recently but i need to look through my notes again

because u need two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei to form isotopes.

so since they all have the same shape? i mean maybe because m& m and reeses are both chocolates