compounds with the same chemical composition may have different densities because?

Question

they...
a) have differences in reactivity

b) are able to bond with oxygen

c) vary in solubility

d) exist in different places

Answer 1

I think that the answer is d), although it should read "different phases"

Answer 2

I would think the answer would be: because they are different allotropes or of different crystal structures.

Think of carbon as graphite, charcoal, glassy carbon, & diamond. Or cubic silicon carbide vs hexagonal SiC, cubic vs hexagonal boron nitride etc.