You are correct, the answer is A) Low generator bearing lubricating oil pressure. The reasoning is that in a directly coupled system, the prime mover (engine) is directly connected to the generator. If the generator bearing lubricating oil pressure drops, it can cause damage to the bearings, which would then require the prime mover to automatically shut down to prevent further damage. The other options are incorrect because they do not directly relate to a condition that would automatically shut down the prime mover in a directly coupled system. High generator cooling air temperature, low generator output frequency, and high generator power factor may indicate issues, but they would not necessarily trigger an automatic shutdown of the prime mover.