**Answer:**

In a CMOS (Complementary Metal-Oxide-Semiconductor) inverter, one transistor is an N-channel MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) and the other transistor is a P-channel MOSFET. This arrangement allows for complementary operation, where one transistor is active when the other is off.

**Explanation:**

A CMOS inverter is a fundamental building block in digital integrated circuits. It is used to convert logic levels and perform logic inversion. The two transistors in a CMOS inverter work in tandem to achieve this functionality.

1. **N-Channel MOSFET (NMOS):** The N-channel MOSFET is an enhancement type transistor. It consists of a P-substrate with an N-type channel region. When a positive voltage is applied to the gate terminal, the transistor turns on, allowing current to flow from the drain to the source. In a CMOS inverter, the N-channel MOSFET is often referred to as the "pull-down" transistor.

2. **P-Channel MOSFET (PMOS):** The P-channel MOSFET is a depletion type transistor. It consists of an N-substrate with a P-type channel region. When a negative voltage is applied to the gate terminal, the transistor turns on, allowing current to flow from the source to the drain. In a CMOS inverter, the P-channel MOSFET is often referred to as the "pull-up" transistor.

**Complementary Operation:**

The CMOS inverter operates based on the principle of complementary operation. When the input voltage (VIN) is high (logic 1), the N-channel MOSFET turns off, while the P-channel MOSFET turns on. This allows the output voltage (VOUT) to be low (logic 0). Conversely, when the input voltage (VIN) is low (logic 0), the N-channel MOSFET turns on, while the P-channel MOSFET turns off. This allows the output voltage (VOUT) to be high (logic 1).

**Advantages of CMOS Inverter:**

The CMOS inverter offers several advantages, including:

- Low power consumption: CMOS transistors consume power only when they switch states, resulting in minimal power dissipation.
- High noise immunity: The complementary operation of the CMOS inverter helps to minimize the susceptibility to noise and interference.
- Wide operating voltage range: CMOS inverters can operate over a wide range of supply voltages.
- High packing density: The small size of CMOS transistors allows for high-density integration of circuits on a chip.

In conclusion, the correct answer is option 'B' because one transistor in a CMOS inverter is an enhancement type (N-channel) MOSFET, and the other transistor is a depletion type (P-channel) MOSFET.