The transformer ratings are usually expressed in terms of

The correct answer is D. kVA.

A transformer is a device that transfers electrical energy from one circuit to another without a change in frequency. The transformer ratings are usually expressed in terms of kVA, which is the product of the rated voltage and the rated current. The kVA rating of a transformer is a measure of its power handling capacity.

The other options are incorrect because they are not measures of power handling capacity. Volts are a measure of electrical potential difference, amperes are a measure of electrical current, and kW is a measure of electrical power.