A heat engine uses thermal energy flowing from a heat source that is 225 K to a cold sink that is 50 K.

For an ideal heat engine, the maximum efficiency is 1 − (cold-sink temperature ÷ heat-source temperature). Substituting the temperatures gives 1 − (50 K ÷ 225 K) ≈ 0.778, or about 78%. Therefore, the correct choice is C) 78%.

AI-written and checked against the verified answer.