One mole of an ideal gas goes from an initial state A to final state B via two processes. It first undergoes isothermal expansion from volume V to 3V and then its volume is reduced from 3V to V at constant pressure. The correct P-V diagram representing the two processes is:

A process in which the pressure remain constant is isobaric process. And PV = nRT

 As P = constant ; V ∝ T

1st process is V → 3V (Isothermal)

2nd process is 3V → V (Isobaric)