The correct answer is: D. 50% load.
Distribution transformers are generally designed for maximum efficiency around 50% load. This is because the losses in a transformer are proportional to the square of the current. When the transformer is loaded at 50%, the current is half of the full-load current, and the losses are therefore one-quarter of the full-load losses.
At 0% load, there is no current flowing through the transformer, and the losses are therefore zero. However, there are still some losses due to core magnetization and eddy currents. These losses are relatively small, but they can still account for a significant portion of the total losses at low loads.
At 100% load, the current is twice the full-load current, and the losses are therefore four times the full-load losses. The losses are therefore much higher at 100% load than at 50% load.
For these reasons, distribution transformers are generally designed for maximum efficiency around 50% load.