Introduction:
A fully controlled converter is a power electronic device used for converting alternating current (AC) to direct current (DC) or vice versa. It is used in various applications such as motor drives, power supplies, and renewable energy systems. The converter can be controlled to vary the output voltage or current according to the desired requirements.

Explanation:
A fully controlled converter uses thyristors, also known as silicon-controlled rectifiers (SCRs), as the main switching devices. Thyristors are semiconductor devices that can handle high voltage and current levels and can be triggered to conduct current when a specific control signal is provided.

Advantages of Thyristors:
- Thyristors have the ability to handle high power levels and can operate at high temperatures.
- They have a high blocking voltage capability, making them suitable for high voltage applications.
- Thyristors are capable of handling both AC and DC currents.
- They have a high current carrying capacity, which makes them suitable for high power applications.
- Thyristors provide a high level of control over the output voltage or current.

Fully Controlled Converter Operation:
In a fully controlled converter, thyristors are used in pairs to form a bridge configuration. The bridge consists of four thyristors connected in a specific arrangement. By controlling the firing angle of the thyristors, the output voltage or current can be controlled.

When the thyristors are triggered, they conduct current in one direction, allowing the flow of current from the AC source to the load. By varying the firing angle, the amount of time the thyristors conduct current can be controlled, thereby controlling the output voltage or current.

Other Components:
While thyristors are the main switching devices in a fully controlled converter, other components such as diodes, capacitors, and inductors are also used to provide the required functionality. Diodes are used to rectify the AC input voltage, while capacitors and inductors are used for filtering and smoothing the output waveform.

Conclusion:
In conclusion, a fully controlled converter uses thyristors as the main switching devices. Thyristors provide high power handling capabilities and allow for precise control of the output voltage or current. Other components such as diodes, capacitors, and inductors are also used to achieve the desired functionality of the converter. Therefore, the correct answer is option 'B' - thyristors only.