To understand why the correct answer is option 'B', let's first discuss what a thyristor is and its operating principle.

Introduction to Thyristor:
A thyristor is a four-layer, three-junction semiconductor device that acts as a switch in electronic circuits. It is a unidirectional device, meaning it can only conduct current in one direction. The thyristor has three terminals - anode, cathode, and gate. The anode is the positive terminal, the cathode is the negative terminal, and the gate is used to control the switching action of the thyristor.

Operating Principle:
The thyristor operates in two modes - off state and on state. In the off state, the thyristor behaves like an open switch and does not conduct current. In the on state, the thyristor behaves like a closed switch and allows current to flow through it.

To initiate conduction in a thyristor, a gate current is required. This gate current triggers the thyristor into the on state. Once triggered, the thyristor remains in the on state even if the gate current is removed until the forward voltage across it drops below a certain threshold.

Explanation of the Correct Answer:
The correct answer to the question is option 'B' - forward breakover voltage. The forward breakover voltage is the minimum voltage that must be applied across the thyristor in the forward direction to trigger it into the on state.

When the forward voltage across the thyristor exceeds the forward breakover voltage, it triggers the thyristor into the on state. At this point, the thyristor starts conducting current and remains in the on state until the forward current drops below a certain threshold called the holding current.

So, in order to start conduction in a thyristor, the forward voltage must exceed the forward breakover voltage. Once triggered into the on state, the thyristor will continue to conduct current until the forward current drops below the holding current.

Conclusion:
In summary, a thyristor requires a forward voltage greater than the forward breakover voltage to initiate conduction. The gate current is only required to trigger the thyristor into the on state but is not necessary to start conduction once the thyristor is triggered.