The correct answer is option 'C' - 4 thyristors.

A fully controlled bridge rectifier is a circuit configuration used to convert alternating current (AC) into direct current (DC) using thyristors. Thyristors are semiconductor devices that can control the flow of current in a circuit. In a single-phase fully controlled bridge rectifier, there are four thyristors used.

Let's understand why there are four thyristors in this rectifier configuration:

1. **Bridge Rectifier Configuration**: A bridge rectifier uses a combination of diodes to convert AC to DC. In a fully controlled bridge rectifier, the diodes are replaced with thyristors to allow for control of the output. The configuration consists of four arms, each containing a thyristor and a diode.

2. **Working Principle**: The AC input is connected to the center tap of a transformer, and the two outer terminals of the secondary winding are connected to the anodes of the thyristors. The cathodes of the thyristors are connected to the positive and negative terminals of the DC output. The junction between the thyristor and the diode in each arm is the output of the rectifier.

3. **Operation**: To convert AC to DC, the thyristors are triggered in pairs, called a positive and negative pair. When a positive pair is triggered, it allows current to flow in one direction, and when the negative pair is triggered, it allows current to flow in the opposite direction. By controlling the triggering of the thyristors, the output voltage and current can be controlled.

4. **Number of Thyristors**: In a single-phase fully controlled bridge rectifier, each arm requires one thyristor. Since there are four arms in the bridge configuration, there are a total of four thyristors used.

Therefore, the correct answer is option 'C' - 4 thyristors. These four thyristors work together to control the flow of current and convert AC to DC in a single-phase fully controlled bridge rectifier circuit.