Introduction:
The waveguide is an important component in the field of electrical engineering, particularly in the transmission of electromagnetic waves. It is widely used in various applications such as communication systems, radar systems, and microwave devices. The choice of operating frequency for a waveguide depends on several factors, including the desired transmission characteristics, power handling capability, and size constraints.

Explanation:
The correct answer to the given question is option 'D', which indicates that the waveguide is employed in the transmission lines when operated at the range of GHz (Gigahertz). Let's understand why this is the correct choice.

Frequency Range:
The frequency range of a waveguide refers to the range of frequencies at which it can efficiently transmit electromagnetic waves. This range is determined by the dimensions of the waveguide and its mode of operation. Different types of waveguides support different frequency ranges.

Waveguide Operation:
A waveguide is a hollow metallic structure that guides and propagates electromagnetic waves along its length. It consists of a metal tube or channel with a specific cross-sectional shape, such as rectangular, circular, or elliptical. The dimensions of the waveguide are designed to support a particular range of frequencies.

Operating Frequency:
The operating frequency of a waveguide is the frequency at which it is intended to be used. The choice of operating frequency depends on the specific application requirements. For example, in microwave communication systems, the waveguide is commonly used in the GHz range.

Advantages of GHz Range:
Operating the waveguide in the GHz range offers several advantages:

1. Higher data rates: The GHz frequency range allows for higher data transmission rates, making it suitable for high-speed communication systems.

2. Improved signal quality: The GHz range provides better signal quality with reduced interference and noise compared to lower frequency ranges.

3. Miniaturization: GHz frequency signals can be used in compact waveguide designs, allowing for miniaturization of devices.

4. Compatibility with other components: Many microwave devices and components, such as antennas and amplifiers, are designed to operate in the GHz range, making it convenient to use a waveguide in the same frequency range.

5. Efficient transmission: The waveguide exhibits low loss and high power handling capability in the GHz range, ensuring efficient transmission of electromagnetic waves.

Conclusion:
In conclusion, the waveguide is employed in the transmission lines when operated in the GHz range. This frequency range offers several advantages, including higher data rates, improved signal quality, miniaturization, compatibility with other components, and efficient transmission. The selection of the operating frequency depends on the specific application requirements and the characteristics of the waveguide.