$\mathrm{ABCA}$ is a cyclic process in which 

$\mathrm{AB}$ Process is isobaric as in $\mathrm{AB}$ Process volume is directly proportional to temperature 

$\mathrm{BC}$ is isochoric process as volume is constant 

$\mathrm{CA}$ is isothermal process as temperature is constant in it .

For isothermal process $P-V$ graph is rectangular hyperbola