C (straight line) of the pV diagram is wrong.
Work Step by Step
[a] represents an Isobaric process -constant pressure. An isobaric process can be seen as a horizontal line on a pV diagram. [b] represents an Isochoric process - constant volume. It can be seen as a vertical line on a pV diagram. [c] is a straight line on the pV diagram. This straight line does not represent an isothermal process. An isothermal process is represented as a hyperbola on a pV diagram and not a straight line. Therefore C of the diagram is wrong.