$V\propto {T^{\frac{2}{3}}}$ $[R = 1.99\ cal/mol-K]$
$A \rightarrow B :$ Isothermal expansion at temperature $T$ so that the volume is doubled from $V _{1}$ to $V _{2}=2 V _{1}$ and pressure changes from $P _{1}$ to $P _{2}$
$B \rightarrow C :$ Isobaric compression at pressure $P _{2}$ to initial volume $V _{1}$
$C \rightarrow A$ : Isochoric change leading to change of pressure from $P _{2}$ to $P _{1}$
Total workdone in the complete cycle $ABCA$ is

$1.$ efficiency more than $27 \%$
$2.$ efficiency less than the efficiency a Carnot engine operating between the same two temperatures.
$3.$ efficiency equal to $27 \%$
$4.$ efficiency less than $27 \%$