Answer: Option (d) is the correct answer.
Explanation:
Atomic number is defined as the total number of protons present in an atom. Whereas mass number is the sum of total number of protons and neutrons present in an atom.
For example, atomic number of nitrogen is 7, this also means that there are 7 protons in a nitrogen atom.
Thus, we can conclude that atomic number is the number of an atom's protons.
