Detailed Explanation:

Isotopes are atoms of the same element with identical atomic numbers (same number of protons) but different mass numbers due to varying numbers of neutrons. For example, Carbon-12 and Carbon-14 both have 6 protons but differ in neutron count (6 vs 8), resulting in different mass numbers (12 vs 14).

The number of protons and electrons remains constant within isotopes of an element, as changing protons would create a different element entirely, and neutral atoms maintain equal protons and electrons.

Therefore, isotopes differ only in mass number, making Option 2 the correct answer.