For isothermal process $T$ is constant

So $\Delta U =0$

$A \longrightarrow II$

Adiabatic process

$\Delta Q =0$

$\Delta Q =\Delta U +\Delta W$

$\quad \Delta U =-\Delta W$

Work done by gas is positive

So $\Delta U$ is negative

$B \longrightarrow I$

For Isochoric process $\Delta W =0$

$C \longrightarrow IV$

For Isobaric process

$\Delta W = P \Delta V \neq 0$

$\Delta U = nC _{ V } \Delta T \neq 0$

Heat absorbed goes partly to increase internal energy and partly do work.