Functions of a Transistor:

A transistor is a three-terminal electronic device that can amplify or switch electronic signals and electrical power. It is a fundamental building block of modern electronic devices and is widely used in various applications. The functions of a transistor include:

1. Variable Resistor:
A transistor can function as a variable resistor when it is operated in its linear region. By applying a small input voltage or current to the base terminal, the transistor can control the amount of current flowing between its collector and emitter terminals. This feature allows the transistor to act as a variable resistor, where the resistance value can be adjusted based on the input signal. This characteristic is commonly used in applications such as audio amplifiers, where the transistor can control the volume of the sound.

2. Switching Device:
A transistor can also function as a switching device. When operated in its cutoff and saturation regions, the transistor can turn ON or OFF the flow of current between its collector and emitter terminals. By applying a small input voltage or current to the base terminal, the transistor can control the switching action. This feature allows the transistor to act as a digital switch, where it can be used to control the state of other electronic devices or circuits. This characteristic is commonly used in applications such as digital logic gates, where the transistor can control the flow of data between different parts of a circuit.

Therefore, the correct answer is option 'D' - Variable resistor and switching device. A transistor can function as a variable resistor in its linear region and as a switching device in its cutoff and saturation regions. It is important to note that a transistor does not function as a rectifier, fixed resistor, or tuning device. These functions are typically performed by other electronic components such as diodes, resistors, and capacitors.