The correct answer is C. near full-load.
Power transformers are generally designed to have maximum efficiency around near full-load. This is because the losses in a transformer are proportional to the square of the current. When the transformer is operating at near full-load, the current is lower than when it is operating at no-load or half-load. This means that the losses are also lower, and the efficiency is higher.
When a transformer is operating at no-load, there is no current flowing through the secondary winding. This means that there is no power being transferred, and the efficiency is zero. When a transformer is operating at half-load, the current is flowing through the secondary winding, but it is not as high as when the transformer is operating at full-load. This means that the losses are higher than when the transformer is operating at no-load, but they are still lower than when the transformer is operating at full-load.
When a transformer is operating at full-load, the current is flowing through the secondary winding at its maximum value. This means that the losses are at their maximum value, and the efficiency is at its minimum value.
In conclusion, power transformers are generally designed to have maximum efficiency around near full-load. This is because the losses in a transformer are proportional to the square of the current. When the transformer is operating at near full-load, the current is lower than when it is operating at no-load or half-load. This means that the losses are also lower, and the efficiency is higher.