The correct answer is A. mass.
In a compound, the elements are always present in definite proportions by mass. This means that the ratio of the masses of the elements in a compound is always the same. For example, water is always made up of two parts hydrogen and one part oxygen by mass. This is true no matter how much water you have.
The other options are incorrect. Option B, volume, is incorrect because the volume of the elements in a compound can vary depending on the conditions. For example, if you heat water, it will expand and its volume will increase. Option C, size, is incorrect because the size of the elements in a compound does not affect the ratio of their masses. Option D, none of these, is incorrect because option A is the correct answer.