Answer:
C. number of protons in an atom's nucleus.
Explanation:
Atomic number : It is defined as the number of electrons or number of protons present in a neutral atom.
However, when we talk about the atomic number of the ion, it is not equal to the number of electrons as electron can be gained or loosed.
This is why, more appropriately, the number of the protons which are present in the nucleus of the atom is called the atomic number.
For example, Sodium has 11 protons in its nucleus which suggest that the atomic number of sodium is 11.
