Answer:B. Neutrons
Explanation: Isotopes are atoms of an element which have the same atomic number (number of protons) but different atomic mass or mass number. Atomic mass or mass number of an atom is the sum of the number of protons and neutrons in the nucleus of an atom. The difference in their mass number is due to the difference in the number of neutrons. A phenomenon whereby there is existence of atoms of the same element which have the same atomic number but different mass number is known as isotopy.
