What device can be used to decrease voltage in a circuit?

A transformer is the device utilized for decreasing voltage in an electrical circuit. It operates on the principle of electromagnetic induction, allowing the voltage to be transformed either upwards or downwards depending on the ratio of the coils in the transformer. When the primary coil receives electrical energy, a magnetic field is generated, which induces voltage in the secondary coil. By adjusting the number of turns of wire in each coil, a transformer can effectively lower the output voltage relative to the input voltage.

Other devices mentioned have distinct functions. A rheostat is primarily a variable resistor used to control current flow, but it does not inherently decrease voltage in a way that can be applied across different load conditions. A capacitor stores electrical energy and can influence the voltage in circuits, but it doesn't actually decrease voltage in the same manner as a transformer. A diode primarily controls the direction of current flow and does not function as a device for decreasing voltage. Thus, the transformer is uniquely suited for this purpose, making it the correct answer.