This is because the unit which is widely used in describing mass in nuclear as well as atomic physics is unified atomic mass unit(amu).

This amu is equal to 1/12^th of the mass of a carbon-12 isotope. Thus for a carbon atom in the ground state, the mass will be equal to 12amu.

Option(b) is not appropriate as because the mass will not be less than 12u (as it is exactly equal to 12 u).

Option(c) is also not appropriate as the mass cannot be more than 12u (as it is exactly equal to 12 u).

Option(d) is not appropriate as the definition describes that the mass is equal to 1/12^th of carbon-12 isotope. So, it will not depend upon graphite or charcoal.