Which of the following is not a transmission media ?

The correct answer is (d). A modem is a device that converts digital signals to analog signals and vice versa, so it is not a transmission medium.

A transmission medium is a physical path or channel that carries a signal from one location to another. Examples of transmission media include:

• Telephone lines: These are used to carry voice and data signals over long distances.
• Coaxial cables: These are used to carry high-speed data signals.
• Microwave systems: These are used to carry data signals over long distances using radio waves.
• Fiber optic cables: These are used to carry high-speed data signals using light.

Modems are used to connect computers to the internet. They convert digital signals from computers to analog signals that can be transmitted over telephone lines, and vice versa.