An element's atomic mass does not include the mass of its what

Electrons

Explanation:

Atomic mass does not include the weight of the atoms electrons. This is because the weight of an electron is so small, even if it was added to the overall mass it would make very little to no difference.

a and b because with the other three, they simplify to something else.

answer(c) carved on a babylonian clay tablet that showed earth as a flat circular disk;