an isotope of an element is the same element (same atomic number) but with a different number of protons…

Brief Explanations:

Isotopes of an element have the same atomic number (so same number of protons). The atomic number defines the element. The mass number (protons + neutrons) can differ. Since protons are same (as atomic number is same), the difference in mass number comes from different number of neutrons. Electrons can vary in ions (not in isotopes). Quarks are sub - particles of protons and neutrons and not relevant in this context.

Answer:

neutrons.