A heat engine has a temperature of 1800 K. Some of the heat from the engine flows to the surroundings, which are at a temperature of 160 K.

The maximum possible efficiency of a heat engine operating between two temperatures is η = 1 − T_cold/T_hot. Using the temperatures in kelvins, η = 1 − 160/1800 ≈ 0.911, or about 91%. So the correct choice is 91%.

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