Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.

Here, A is a mass number and Z is an atomic number of X(any element).

From the above notation, to be the pair of isotope Z should be similar. Only A and B have pairs of isotopes and one the other hand, C and D have different atomic number.