Which component is used as a voltage regulator?

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Voltage regulation in simple circuits relies on a device that holds the output steady even as input or load current changes. A Zener diode fits that role because it is designed to conduct in reverse at a specific breakdown voltage. When reverse-biased with enough current, it clamps the voltage to its rated Zener voltage, keeping the output roughly constant. In a basic regulator, a resistor feeds current into the Zener; as the load draws varying current, the Zener current adjusts to maintain near-constant voltage, within its rated range.

A resistor by itself merely drops voltage proportional to current; a capacitor stores charge but does not regulate voltage, it only filters or smooths. A regular diode conducts in forward direction with a roughly constant forward drop, but that drop varies with current and temperature and does not provide stable regulation.