An atom of the same element that differs by the number of neutrons is called:

Concept: The basic identity of an atom or element is determined by the number of neutrons, protons, and electrons.

Isotopes are atoms with the same atomic number but different mass numbers. For example, the hydrogen atom has three isotopes, each of which differs by one in its mass number. 
So, the number of neutrons, despite being neutral, is important in determining the mass number of an atom.

So, an atom of the same element that differs by the number of neutrons is called an isotope.

Hence, option D is correct.