Break the identity of an atom into two separate numbers.
- Atomic number (Z) equals the number of protons. This number defines which element the atom is. Change it, and you get a completely different element.
- Mass number (A) equals protons plus neutrons. This number can vary among atoms of the very same element.
Isotopes keep Z fixed, same element, same proton count, and since a neutral atom has electrons equal to protons, the same electron count too, but let the neutron count vary, which changes A. Option A in the question is simply the definition of a neutral atom and has nothing special to do with isotopes. Option C wrongly swaps protons and neutrons. Option D wrongly demands an equal nucleon count, which is the opposite of what makes isotopes different. Only option B correctly states that isotopes share the same proton count but differ in neutron count.