A decrease in it's operating temperature would make a heat engine less efficient. This is because in order to operate, a heat engine needs to be hot and maintain that temperature.
The scientists describe them as D. Elliptical
m = Q(on moon) * G(on moon) = 200N * 1.63N/kg = 326kg
Q(Earth)= g * m = 10m/s2 * 326kg = 3260N