Question

isotopes of an element differ in the number of
energy shells.
protons.
nuclei.
electrons.
neutrons.

Explanation:

Isotopes of an element have the same number of protons (which defines the element), but different numbers of neutrons. The number of electrons is the same as protons in neutral atoms and is not what differentiates isotopes. Energy - shells depend on the electron configuration which is related to the number of protons and is the same for isotopes. Nuclei are the central part of the atom containing protons and neutrons, but the key differentiator is the neutron number among isotopes.

Answer:

E. neutrons