$\mathrm{AB}$ is isothermal reversible expansion process i.e. $\mathrm{\Delta T}=0$ and $\mathrm{S}$ increases as there is increase in volume.

$\mathrm{BC}$ is adiabatic reversible expansion process $\left({\mathrm{q}}_{\mathrm{rev}}=0\right)$ i.e. temperature decreases and $\mathrm{\Delta S}=0$

$\mathrm{CD}$ is isothermal reversible compression process i.e. $\mathrm{\Delta T}=0$ and $\mathrm{S}$ decreases as there is decrease in volume.

$\mathrm{DA}$ is adiabatic reversible compression process $\left({\mathrm{q}}_{\mathrm{res}}=0\right)$ i.e. temperature decreases and $\mathrm{\Delta S}=0$