ANS:D - 70%

Explanation: The thermal efficiency of diesel engines is about 30-50%. This means that for every 100 units of fuel energy input, the engine is able to convert 30-50 units of that energy into useful work. The remaining energy is lost as heat or exhaust. The maximum theoretical efficiency of a diesel engine is about 70%. However, this is rarely achieved in practice due to factors such as friction, heat loss, and incomplete combustion.