Explain how the isotopes of an element differ.
they each have a different number of electrons in their outermost energy level.
they each have a different number of protons in their nuclei.
they each have a different number of neutrons in their nuclei.
they each have a different charge.

Option C, They each have a different number of neutrons in their nuclei.

Explanation:

Isotopes of an element have different number of neutron but they have same number of proton.

For example -

There are two isotopes of carbon i.e carbon-12 and carbon-14

The number of protons in both carbon-12 and carbon-14 is equal to 6

while the number of neutron in carbon-12 is 6 while the number of neutron in carbon-14 is 8

Hence, option C is correct

the answer would be d