A transformer transforms

voltage
current
power
frequency

The correct answer is: A. voltage

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 voltage in the second coil is proportional to the voltage in the first coil, and the number of turns in each coil.

A transformer can be used to increase or decrease voltage. To increase voltage, the second coil has more turns than the first coil. To decrease voltage, the second coil has fewer turns than the first coil.

Transformers are used in a variety of applications, including power transmission, power distribution, and electronics.

Option B, current, is incorrect because a transformer does not change the current. The current in the second coil is equal to the current in the first coil.

Option C, power, is incorrect because a transformer does not change the power. The power in the second coil is equal to the power in the first coil.

Option D, frequency, is incorrect because a transformer does not change the frequency. The frequency in the second coil is equal to the frequency in the first coil.