Answer
C
Work Step by Step
From the definitions:
$\eta_{rev}=1-\dfrac{T_L}{T_H}$
With $T_H=1500\ K,\ T_L=300\ K$, then $\eta_{rev}=80\%$
$\eta_{actual}=\dfrac{\dot{W}}{\dot{Q}_{in}}$
With $\dot{W}=400\ kW,\ \dot{Q}_{in}=600\ kJ/s$, then $\eta_{actual}=66.7\%$
Finally:
$\eta_{ _{II}}=\dfrac{\eta_{actual}}{\eta_{rev}}=83.3\%$
