Question

isotopes are:
atoms of the same element that have different number of electrons.
atoms of the same element that have different number of protons.
atoms of the same element that have different number of neutrons.
atoms of the same element that have the same number of neutrons.
none of the above

Explanation:

Isotopes are defined as atoms of the same element (so same number of protons) with different numbers of neutrons. Atoms of the same element with different electrons are ions. Atoms of the same element cannot have different proton numbers (as proton number defines the element). And having the same number of neutrons would not be isotopes.

Answer:

atoms of the same element that have different number of neutrons.