The primary function of a transformer is:

The primary function of a transformer is to increase or decrease voltage levels in an electrical circuit. This is achieved through the principles of electromagnetic induction, where alternating current (AC) flowing through a coil generates a magnetic field that induces a voltage in another coil. Depending on the configuration of the coils (the primary and secondary windings), transformers can step up (increase) or step down (decrease) voltage.

In practical applications, this ability to adjust voltage levels is crucial for efficiently transmitting electrical power over long distances. Higher voltages reduce energy losses due to resistance in the transmission lines, while lower voltages are suitable for end-user applications.

The other options do not capture the main function of a transformer. For example, converting AC to DC involves rectification processes that transformers do not perform; storing electrical energy typically relates to capacitors or inductors, not transformers; and while transformers can influence current flow due to their effect on voltage, they do not primarily regulate current flow directly. Thus, option B is the most accurate description of a transformer's primary function.