Final answer:
To increase the maximum theoretical efficiency of a heat engine, you should increase the temperature of the hot reservoir and decrease the temperature of the cold reservoir.
Step-by-step explanation:
To increase the maximum theoretical efficiency of a heat engine, we need to consider the temperature of the hot and cold reservoirs. Increasing the temperature of the hot reservoir (TH) would increase the efficiency. Decreasing the temperature of the cold reservoir (TC) would also increase the efficiency. By increasing the temperature difference between the hot and cold reservoirs, we can increase the area inside the closed loop on the PV diagram, making it easier to divert heat transfer to work.