Which one of the following is generally used to increase or decrease voltage?

The transformer is a device specifically designed to change the voltage levels in alternating current (AC) electrical systems. It operates on the principle of electromagnetic induction, where an alternating current in one coil (the primary winding) generates a magnetic field that induces a current in another coil (the secondary winding). By varying the number of turns in each winding, the transformer can increase (step up) or decrease (step down) voltage.

This makes it essential for applications such as power transmission, where high voltage is needed to reduce energy loss over long distances, as well as for voltage reduction to safe levels for consumer use.

The other choices serve different purposes: a rectifier converts alternating current to direct current, a voltmeter measures voltage rather than modifying it, and a shunt is used to measure current by providing a low-resistance path. Hence, these options do not perform the function of altering voltage levels like a transformer does.