What will be the least rate of heat rejection per kW net output of the engine?

Question

In a heat engine operating in a cycle between a source temperature of 606°C and a sink temperature of 20°C, what will be the least rate of heat rejection per kW net output of the engine?      

a. 0.50kW                    b. 0.667 kW

c. 1.5 kW                     d. 0.0341 kW