The correct answer is C) kilovolt amperes. Power transformers are rated in kilovolt amperes (kVA) because this unit of measurement combines both the voltage (kilovolts) and the current (amperes) to provide a complete representation of the transformer's power capacity. The kVA rating is the standard method used in the electrical industry to specify the size and capacity of power transformers. The other options are incorrect because they do not fully capture the power rating of a transformer. Kilowatt-volts (option A) and ampere-turns (option D) are not commonly used to rate power transformers, while kilowatts-amps (option B) is an incorrect combination of units.