A 3-phase AC regulator is a device used to control the output voltage of a 3-phase AC system. It is commonly used in industrial applications to regulate the voltage supplied to motors and other electrical equipment. Thyristors, also known as silicon-controlled rectifiers (SCRs), are electronic devices that can control the flow of current in a circuit. In the case of a 3-phase AC regulator, thyristors are used to control the firing angle of the AC waveform, which in turn controls the average voltage applied to the load.

The correct answer is option 'B', which states that a 3-phase AC regulator uses 6 thyristors. This is because a 3-phase system consists of three separate phases, each with its own voltage waveform. To regulate the voltage in each phase, two thyristors are required per phase. Therefore, in total, 6 thyristors are needed to regulate the voltage in a 3-phase AC system.

Let's break down the answer further:

1. Basics of a 3-phase AC system:
- A 3-phase AC system consists of three separate phases: phase A, phase B, and phase C.
- Each phase has a voltage waveform that is 120 degrees out of phase with the other phases.

2. Purpose of a 3-phase AC regulator:
- A 3-phase AC regulator is used to control the output voltage of a 3-phase AC system.
- It allows for precise control of the voltage supplied to motors and other electrical equipment.

3. Thyristors in a 3-phase AC regulator:
- Thyristors, or SCRs, are used to control the firing angle of the AC waveform in each phase.
- By controlling the firing angle, the average voltage applied to the load can be regulated.

4. Number of thyristors required:
- To regulate the voltage in each phase, two thyristors are required per phase.
- This is because each thyristor can only control the current flow in one direction.
- By using two thyristors in an anti-parallel configuration, the current can be controlled in both directions.

5. Total number of thyristors:
- Since a 3-phase AC system has three separate phases and each phase requires two thyristors, the total number of thyristors required is 6.

In conclusion, a 3-phase AC regulator requires 6 thyristors, with two thyristors per phase, to control the voltage in each phase of a 3-phase AC system.