The correct answer is option 'D', radio waves.

TV remote control units use radio waves to operate a TV set. Radio waves are a type of electromagnetic radiation that have a longer wavelength and lower frequency than visible light. They are commonly used for wireless communication, including remote control devices.

Radio waves are transmitted from the remote control unit to the TV set, carrying signals that correspond to specific commands. These signals are encoded and modulated onto the radio waves, allowing the TV set to receive and interpret them.

Below are the key points explaining why TV remote control units use radio waves:

- Radio waves are ideal for remote control devices because they can travel through walls, furniture, and other obstacles without significant loss of signal strength. This allows users to control their TV sets from a distance and without having to point the remote directly at the TV.

- Radio waves have a longer range compared to other types of electromagnetic waves, such as light waves or sound waves. This allows users to control their TV sets from different areas of a room or even from another room.

- Radio waves can carry complex signals that can encode various commands and functions. This enables remote control units to have multiple buttons and functions, allowing users to control various aspects of their TV sets, such as changing channels, adjusting volume, and accessing different menus.

- TV remote control units use specific frequencies within the radio wave spectrum to transmit their signals. These frequencies are carefully chosen to minimize interference from other devices that may also use radio waves, such as Wi-Fi routers or cordless phones.

- The TV set is equipped with a receiver that is designed to pick up the radio wave signals sent by the remote control unit. This receiver demodulates and decodes the signals, allowing the TV set to respond accordingly.

In conclusion, TV remote control units use radio waves to operate a TV set due to their ability to transmit signals over long distances, penetrate obstacles, and carry complex commands.