Why are the elements in a compound in a fixed proportion? a. the greater the volume, the more there will be of one element in the compound. b. an increase in temperature will increase the volume of all elements in the compound. c. the density of the elements will never change. d. the proportion of each element will remain constant, no matter how large or small the volume.

I believe that the correct answer among the choices is:

D. The proportion of each element will remain constant, no matter how large or small the volume.

 

The proportion of the elements is an inherent property of a compound. It can never be changed no matter the size of the compound. This is because the elements are linked by chemical bonds.

The correct option is D.
The law of fixed proportion states that a given chemical compound always contains its component elements in fixed ratio by mass. This law implies  that a particular chemical compound will always be made up of the same proportion of elements. For instance,a molecule of water is made up of two hydrogen atoms and one oxygen atom. No mater the amount of water that is involved, the ratio of combination of the elements in the water remains the same. The law of fixed proportion, thus ensure that the proportion of elements in compounds remain the same no matter their quantities. 

It’s the energy source of stars

Well technically there are two correct answers b and d. but for you its probably b in general.good day sir/ma'am : ) (i know not gender)