Explanation

(a) In p-V diagrams process AB is isobaric process in which pressure remains constant ie, p=constant at all temperatures.
Process BC is isothermal process in which, temperature remains constant ie, T=constant.
Process CD is isochoric process in which volume remains constant ie, p-T diagram CD is a straight line passing through origin.
Process AD is adiabatice process which corresponds to process AD in p-T diagram.
Hence, the correct p-T diagram is shown in option (a).