Option A is the correct statement about Zener diode. It works as a voltage regulator in reverse bias and behaves like a simple pn junction diode in forward bias. When a Zener diode is reverse-biased, it operates in the breakdown region, where a relatively constant voltage is maintained across the diode, regardless of the current flowing through it. This property makes it useful as a voltage regulator. In forward bias, the voltage applied across the diode is in the same direction as the normal direction of current flow. In this condition, the Zener diode behaves like a simple pn junction diode and allows current to flow in the forward direction.