Answer:
The correct answer would be A. numbers of neutrons.
Isotopes refer to the types or forms of the same element which contain same numbers of protons but different numbers of neutrons due to which they differ in mass number.
For example, carbon 14 and carbon 12 are isotopes of carbon. Carbon 14 contains 8 neutrons whereas carbon 12 contains 6 neutrons.
Similarly, Protium (¹H), deuterium (²H), and tritium (³H) are isotopes of hydrogen.
