Concept:The mass of a nucleus depends on the nucleons inside it, not on electrons.
Explanation:A nucleus contains protons and neutrons, which are collectively called nucleons.
Electrons orbit outside the nucleus and have a very tiny mass compared to nucleons.
So the mass of a nucleus is almost entirely the sum of the masses of its protons and neutrons.
In exam terms, this is expressed as the total number of protons and neutrons in the nucleus, also known as the mass number.
Option B refers only to protons, and option C refers to electrons, which do not contribute significantly to nuclear mass.
Option D is incorrect because electrons are not part of the nucleus.
Hence, the correct choice is the option that states the total number of protons and neutrons.
Answer:A. total number of its proton & neutrons.