Process ab is adiabatic expansion. So pressure in the process decreases as P=Vγk,(γ>1) .
Process bc is straight line passing through origin that means T=kV. From ideal gas equation, T=(nRP)V
So pressure during the process must be constant.
Process ca is isothermal. Final p−V graph will have three sections :
(1) isothermal (2) adiabatic (3) isobaric