Answer: d. atomic mass unit.
Explanation: Atomic mass unit is equal to one twelfth of the mass of an atom of carbon-12 isotope.
Atomic mass of an element is the number of times an atom of that element is heavier than an atom of carbon taken as 12.
Mass number is the sum of the number of protons and the number of neutrons. It is always a whole number.
Atomic number is the the total number of protons present in the nucleus or total number of electrons present in the neutral atom. It is always a whole number.
