What is the function of a bulb in an electrical circuit?

The function of a bulb in an electrical circuit is to light up when powered. When an electric current flows through the filament of the bulb, the filament heats up due to its resistance, producing light. This is a practical application of converting electrical energy into light energy, allowing the bulb to serve as a visible indicator of electrical flow.

The other options, such as measuring current, reducing current, or opening the circuit, refer to different components or functions not associated with a bulb. For instance, an ammeter measures current, a resistor can reduce current, and a switch can open a circuit by stopping the flow of electricity. Thus, the bulb's primary role is specifically to illuminate when an electrical current is applied.