Final answer:
A transformer can change both current and voltage, which is achieved through the ratio of winding turns of the primary and secondary coils, obeying the principle of conservation of energy. Option d is the answer.
Step-by-step explanation:
A transformer is an electrical device that can change both current and voltage. Specifically, a transformer operates on the principle of electromagnetic induction to convert voltages from one level to another, and, as a consequence of energy conservation, the current changes inversely if we assume that no energy is lost. For instance, a step-up transformer increases voltage and decreases current, facilitating the transmission of power over long distances with reduced loss, while a step-down transformer does the opposite, making high voltages from power lines suitable for use in homes by reducing them to a more manageable level.
An important aspect of transformer operation is the ratio of the number of turns in the primary coil to the number of turns in the secondary coil. This ratio determines whether the output voltage will be higher, lower, or the same as the input voltage. Consequently, a transformer can adjust voltage and current according to the needs of a particular application, always maintaining the balance where the input power approximately equals the output power, considering transformer efficiency often exceeds 99%.
So, in response to the original question, a transformer can accomplish D. changing both current and voltage.