The correct answer is: A. nearly full load.
Power transformers are designed to have maximum efficiency at nearly full load. This is because the losses in a transformer are proportional to the square of the current. When the transformer is operating at nearly full load, the current is relatively low and the losses are therefore also low. As the load on the transformer decreases, the current decreases and the losses increase. At no load, the current is zero and the losses are therefore at their maximum.
Option B is incorrect because 70% full load is not the point at which transformers have maximum efficiency. Option C is incorrect because 50% full load is not the point at which transformers have maximum efficiency. Option D is incorrect because transformers have minimum efficiency at no load.