The correct answer is C. at or near full load.
A transformer is a device that transfers electrical energy from one circuit to another without a change in frequency. It does this by using electromagnetic induction to create a magnetic field that induces a voltage in a second coil. The efficiency of a transformer is the ratio of the output power to the input power.
The efficiency of a transformer is affected by a number of factors, including the load on the transformer, the type of core material used, and the operating temperature. In general, transformers are most efficient when they are operating at or near full load. This is because the losses due to core magnetization and copper losses are lower at full load.
At one-fourth load, the transformer is not operating at its full potential and the efficiency will be lower. At one-half load, the efficiency is higher than at one-fourth load, but it is still not as high as when the transformer is operating at full load.
Therefore, the correct answer is C. at or near full load.