The correct answer is: A. on full-load.
A transformer’s regulation is a measure of how well it maintains its output voltage as the load changes. The regulation is expressed as a percentage, and a lower regulation indicates a better transformer.
A transformer’s regulation is affected by a number of factors, including the load, the power factor, and the efficiency. The load is the most important factor, and the regulation is typically lowest at full-load. This is because the transformer’s losses are proportional to the square of the current, and the current is highest at full-load.
The power factor is also a factor, and the regulation is typically better at leading power factors. This is because the leading power factor causes the transformer’s losses to be lower.
The efficiency is also a factor, and the regulation is typically better for more efficient transformers. This is because the more efficient transformer has lower losses, and therefore the regulation is lower.
In conclusion, a transformer can have regulation closer to zero on full-load. This is because the load is the most important factor in determining a transformer’s regulation, and the regulation is typically lowest at full-load.