Which statement is true regarding the operation of a transformer?

The correct statement about the operation of a transformer indicates that a transformer is not a constant wattage device. Transformers work on the principle of electromagnetic induction to convert electricity from one voltage level to another while maintaining power consistency. The wattage does not stay constant throughout; instead, the transformer can either step up (increase) or step down (decrease) voltage based on the number of turns in the primary and secondary coils, while keeping the power (watts) approximately equal minus any losses due to inefficiencies.

In terms of operation, as voltage changes, the current also changes inversely to maintain the power balance due to the equation Power = Voltage × Current. This characteristic allows transformers to match different voltage levels depending on the needs of the electrical system, thus demonstrating that wattage is not constant across all operations.

The other statements do not accurately represent the functionality of transformers. For instance, while they are indeed used to step down or step up voltage, this doesn't limit them to only reducing voltage. Additionally, transformers are not considered constant voltage devices because their output voltage can fluctuate based on load conditions. Therefore, while they have key functions relating to voltage transformation, they inherently do not operate at constant wattage.