What makes an isotope of an element?

Question

Different forms of an element have the same number of protons but different numbers of electrons.

Different forms of an element have a different number of protons but the same number of neutrons.

Different forms of an element have the same number of protons but different numbers of neutrons.

Different forms of an element have a different number of protons but the same
number of electrons.

Answer 1

Different forms of an element have the same number of protons but different numbers of neutrons.