Which of the following transformers increases voltage levels?

The option that correctly identifies a transformer designed to increase voltage levels is the step-up transformer. This type of transformer operates on the principle of electromagnetic induction and is specifically constructed to raise the voltage while reducing the current in an electrical circuit.

In a step-up transformer, the primary coil, which receives the input voltage, has fewer turns of wire compared to the secondary coil, which supplies the output voltage. This difference in the number of turns results in a higher voltage being induced in the secondary coil according to the transformer equation, which states that the voltage ratio between the primary and secondary coils is proportional to the turns ratio. Thus, when electricity flows through a step-up transformer, the energy is effectively converted to increase voltage for applications such as long-distance power transmission, where higher voltages are used to reduce energy loss.

Other types of transformers serve different purposes. A step-down transformer decreases voltage levels, a variable transformer allows for adjustable output voltage but does not inherently increase voltage, and a current transformer is utilized primarily for measuring current rather than directly influencing voltage levels.