Answer:
Option C (two isotopes of an element)
Explanation:
An isotope is two or more forms of the same element that contain equal numbers of protons (as well as electrons) but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties. For example :-
Isotopes of carbon ⇒
In the question , both the atoms have same number of protons , but their number of neutrons isn't same. Hence , they are the isotopes of an element.
