Answer: An element with the same number of protons, but a different number of neutrons
Explanation:
The # of protons in an atom is what determines what atom it is (hydrogen has 1 proton, helium has 2 protons, etc ...). You cannot change the number of protons in an atom without changing what element the atom is.
The number of electrons in atoms varies greatly because electrons are constantly gained, lost, and shared during chemical reactions.
An isotope is a variation of the same element (so they must have the same # of protons) that have different masses (and therefore a different number of neutrons).
The answer is the fourth choice, "An element with the same number of protons, but a different number of neutrons"
