Which option describes a similarity and a difference between isotopes of an element?

A) same atomic number; different number of protons

B) same number of protons; different atomic number

C) same atomic number; different mass number

D) same mass number; different atomic number

E) same number of neutrons; same number of protons

The question says.'which option describes a similarity between isotopes of an element. The correct option is C. Isotopes have the same number of atomic number but different mass number. The mass number is the summation of proton and neutron. The number of proton is always constant but that of neutron changes to reflect the existence of isotopes.