Answer: b. the total number of protons and neutrons in the nucleus
Explanation:
Mass number is defined as the sum of number of protons and neutrons that are present in an atom.
Mass number = Number of protons + Number of neutrons
In an atom, when neutrons or protons are lost or gains, it directly affects the mass number of an atom.
Atomic number is defined as the number of protons or number of electrons that are present in an atom.
Atomic number = Number of electrons = Number of protons
Thus the mass number of an element is equal to the total number of protons and neutrons in the nucleus.
