Introduction:
The common drain amplifier, also known as a source follower, is a type of field-effect transistor (FET) amplifier. It is commonly used to provide a high input impedance and low output impedance. The voltage gain of the common drain amplifier can be determined by analyzing its small-signal equivalent circuit.

Explanation:
The voltage gain of a common drain amplifier is determined by the ratio of the output voltage to the input voltage. In the common drain configuration, the input is applied to the gate terminal, and the output is taken from the drain terminal. The source terminal is connected to a fixed reference voltage.

Operating Principle:
To understand the voltage gain of the common drain amplifier, it is important to understand its operating principle. When a small AC signal is applied to the gate terminal, it modulates the channel current flowing through the transistor. This modulated current flows through the load resistor connected to the drain terminal, producing an output voltage across it.

Factors Affecting Voltage Gain:
The voltage gain of a common drain amplifier is affected by several factors, including the transconductance (gm) of the transistor, the load resistor (RL), and the drain resistor (RD). The transconductance represents the ability of the transistor to convert a small change in the input voltage into a corresponding change in the output current.

Analysis of Small-Signal Equivalent Circuit:
To determine the voltage gain, we can analyze the small-signal equivalent circuit of the common drain amplifier. The small-signal equivalent circuit includes the transistor, the load resistor, and the drain resistor. By applying nodal analysis to this circuit, we can derive an expression for the voltage gain.

Voltage Gain Calculation:
The voltage gain (Av) of the common drain amplifier can be calculated using the following equation:

Av = -gm * RL

Where:
- Av is the voltage gain
- gm is the transconductance of the transistor
- RL is the load resistor

Conclusion:
In conclusion, the voltage gain of a common drain amplifier is always slightly less than 1. This is because the transconductance of the transistor is typically less than 1, and the voltage gain is directly proportional to the transconductance. Therefore, the correct answer is option B) 1.