The Correct Answer is Option 'C' - 4 Leads

Explanation:

A silicon-controlled switch (SCS), also known as a thyristor, is a four-layer semiconductor device that acts as a switch. It has four leads or terminals, each serving a specific purpose in its operation.

The Four Leads of a Silicon-Controlled Switch (SCS) are:

1. Anode (A): The anode is the positive terminal of the SCS. It is connected to the P-type region of the device and is responsible for providing the current flow into the thyristor.

2. Cathode (K): The cathode is the negative terminal of the SCS. It is connected to the N-type region of the device and is responsible for providing the current flow out of the thyristor.

3. Gate (G): The gate is the control terminal of the SCS. It is connected to the middle P-type layer of the device. When a positive voltage is applied to the gate, it triggers the thyristor into a conducting state.

4. G1 (Auxiliary Gate): Some SCS devices have an additional terminal called G1 or the auxiliary gate. It is also connected to the middle P-type layer. The G1 terminal helps in improving the turn-off characteristics of the thyristor and provides additional control over its operation.

Function of Each Lead:
- The anode terminal is responsible for providing the current flow into the thyristor.
- The cathode terminal is responsible for providing the current flow out of the thyristor.
- The gate terminal is used to trigger the thyristor into a conducting state.
- The G1 terminal, if present, helps in improving the turn-off characteristics of the thyristor and provides additional control over its operation.

Conclusion:
A silicon-controlled switch (SCS) has four leads, including the anode, cathode, gate, and G1 (auxiliary gate) terminals. These leads play crucial roles in the operation and control of the thyristor.