What role does a capacitor play in an electric circuit?

A capacitor's primary function in an electric circuit is to store and release electrical energy. This capability allows it to smooth out voltage levels, which is essential for maintaining a steady power supply, especially in applications where fluctuations can cause issues. When a capacitor is charged, it holds energy in an electric field, and when the circuit requires more current than is available from the power source, the capacitor discharges this stored energy, helping to maintain consistent voltage levels. This characteristic is particularly valuable in power supply circuits, filtering applications, and signal processing.

The other roles described in the other choices do not accurately reflect the function of a capacitor. While a capacitor can affect voltage stability and energy storage, it does not serve as a power source for the entire circuit, convert alternating current to direct current, or measure the flow of electric current—functions that are typically performed by other components such as power supplies, rectifiers, and ammeters, respectively.