B. Number of protons in an atom's nucleus
Explanation:
An atomic number is the number of protons present inside the nucleus of an atom. The atomic number helps in identification of a chemical element. In an uncharged atom, the atomic number is also equivalent to number of electrons. However, Mass number is the addition of number of protons and neutrons present inside the nucleus of an atom. Isotopes of an atom have same atomic number but different mass number.
