Chlorine, argon, potassium, calcium have numbers of electrons as 17, 18, 19, 20 respectively. Their nucleus contains 18, 22, 20, 20 neutrons respectively. The pair of isobars are

The elements Argon and Calcium contain same mass number but different atomic numbers.  Hence they are set to be isobars.

Ex: ${ }_{18}^{40}{\text{Ar\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}}_{20}^{40}\text{Ca}$ . Number of neutrons should be different in isobars.

For Ca, number of neutrons n = 20;  For Ar, number of neutrons n = 22.