A cyclic process A B C D is shown below in the given P-V diagram....

In $P$ - $V$ diagrams process $A B$ is isobaric process in which pressure remains constant i.e, $P=$ constant at all temperatures. Process $B C$ is isothermal process in which, temperature remains constant i.e., $T=$ constant. Process $C D$ is isochoric process in which volume remains constant i.e., $P-T$ diagram $C D$ is a straight line passing through origin. Process $A D$ is adiabatic process which corresponds to process $A D$ in $P$ - $T$ diagram Hence, the correct $P-T$ diagram is shown in option (a).