Isotopes are different forms from the same element, they have the same number of protons but differ in the number of neutrons in their nuclei, this makes them have different atomic mass, altough their chemical properties remain the same.
Helium contains 2 protons, 2 electrons, and 2 neutrons.
Of the listed options, only c. has a different number of neutrons than the described atom. So this option represents an Isotope of Helium.
