The correct answer is B. 2.
A dead engine is a locomotive that is not in use and is being towed by another locomotive. Dead engines can be attached to a goods train to provide additional power or to help the train to climb steep hills. The number of dead engines that can be attached to a goods train depends on the weight of the train and the steepness of the hill. In general, two dead engines are sufficient to provide the additional power needed to haul a heavy train up a steep hill.
Option A is incorrect because one dead engine is not enough to provide the additional power needed to haul a heavy train up a steep hill. Option C is incorrect because three dead engines are more than necessary to provide the additional power needed to haul a heavy train up a steep hill. Option D is incorrect because four dead engines are too many and would make the train too long and difficult to control.