Process AB : (Pressure is constant)

If T_A = T ⇒ T_B = T/3

So $∆$U = Negative [$\because ∆$U = nC_v$∆$T]

$∆$W = nR$∆$T = Negative

$∆$Q = $∆$U + $∆$W = Negative

Process BC : (Volume is constant)

If  ${\mathrm{T}}_{\mathrm{B}}=\frac{\mathrm{T}}{3}\mathrm{then}{\mathrm{T}}_{\mathrm{C}}=\frac{\mathrm{T}}{9}$

 $∆$U = nC_v$∆$T = Negative

 $∆$W = Zero

 $∆$Q = Negative

 Process C $\to$ D : (Pressure is constant)

If T_C = T/9 then T_D = T

$∆$U = nC_v$∆$T = positive

$∆$W = positive

$∆$Q = positive

Process D $\to$ A :

T_D = T and T_A = T

Hence process is isothermal

$∆$U = 0

$∆$W = negative

$∆$Q = negative