The correct answer is option 'C' - MOSFET.

Explanation:
A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is the fastest switching device among the given options. Let's understand why:

MOSFET Basics:
A MOSFET is a three-terminal device consisting of a gate, source, and drain. It is a voltage-controlled device where the gate-source voltage controls the flow of current between the source and drain terminals. The MOSFET is widely used in various electronic devices and circuits due to its high switching speed, low power consumption, and small size.

Fast Switching Speed:
The switching speed of a device refers to the time taken to transition between the ON and OFF states. In the context of transistors, it refers to the time taken to transition between the saturation and cutoff regions of operation. The faster the switching speed, the quicker the device can respond to input signals and achieve desired output states.

MOSFET vs. Other Devices:
Now let's compare a MOSFET with the other options given:

a) JFET (Junction Field-Effect Transistor):
JFETs are voltage-controlled devices like MOSFETs, but they have a lower switching speed compared to MOSFETs. This is because JFETs are majority carrier devices, meaning the current flow is primarily due to majority carriers (electrons or holes). The presence of majority carriers slows down the switching process.

b) BJT (Bipolar Junction Transistor):
BJTs are current-controlled devices, and their switching speed is slower than MOSFETs. This is because BJTs require a certain amount of time for the charge carriers to diffuse through the base region before they can be collected at the collector terminal. This diffusion process introduces delays and limits the switching speed.

d) Triode:
A triode is a vacuum tube device and is not commonly used in modern electronic circuits. It has a slower switching speed compared to MOSFETs.

Conclusion:
In conclusion, among the given options, the MOSFET is the fastest switching device. Its ability to switch quickly makes it suitable for high-frequency applications, such as switching power supplies, motor control, RF amplifiers, and digital logic circuits.