A power system comprises a generator, transmission lines, transformers, and loads. Which of the following best describes the function of a transformer in this system?

A$)$ To convert electrical energy into mechanical energy.

B$)$ To step up or step down voltage levels for efficient transmission.

C$)$ To regulate the frequency of the electrical waveform.

D$)$ To convert direct current $($DC$)$ into alternating current $($AC$).$