Introduction:
Zener diodes are special types of diodes that are designed to operate in the reverse breakdown region. They have a unique characteristic of allowing current to flow in the reverse direction when the voltage across them reaches a certain value, known as the Zener voltage. This property makes them useful in various applications, such as voltage regulation and voltage reference circuits.

Explanation:
The correct answer to the given question is option 'A', which states that Zener diodes allow a current to flow in the reverse direction when the voltage reaches above a certain value. Let's understand why this is the correct answer.

Zener Breakdown:
Zener breakdown is a phenomenon that occurs in Zener diodes when they are operated in reverse bias. When a Zener diode is reverse biased, a high electric field is developed across the depletion region. This electric field is strong enough to pull the valence electrons from the valence band of the P-type material and allow them to tunnel into the conduction band of the N-type material. This tunneling effect creates a large number of charge carriers, leading to a sudden increase in current.

Zener Voltage:
The voltage at which this breakdown occurs is known as the Zener voltage (Vz). Every Zener diode has a specified Zener voltage, which is usually marked on its package. When the applied reverse voltage across the Zener diode reaches or exceeds its Zener voltage, the diode starts conducting in the reverse direction.

Current Flow:
Once the Zener voltage is reached, the Zener diode starts conducting a significant amount of current in the reverse direction. This current is often referred to as the Zener current. The Zener diode acts as a voltage regulator by maintaining a relatively constant voltage across it, even when the applied voltage changes.

Applications:
Zener diodes find applications in various electronic circuits. Some of the common applications include:
- Voltage regulation: Zener diodes are used as voltage regulators to maintain a stable voltage output.
- Voltage reference: Zener diodes can be used as a stable voltage reference in precision circuits.
- Overvoltage protection: Zener diodes can protect sensitive components from overvoltage conditions by conducting excessive current in the reverse direction.
- Waveform clipping: Zener diodes can be used to clip or limit the amplitude of an input waveform.

Conclusion:
In conclusion, Zener diodes allow current to flow in the reverse direction when the voltage across them reaches a certain value, known as the Zener voltage. This unique characteristic makes them useful in various applications, such as voltage regulation, voltage reference, overvoltage protection, and waveform clipping.