The correct answer is (c). A modem is a device that modulates and demodulates signals, allowing data to be transmitted over a communication channel. It is not a transmission medium.
A transmission medium is a physical path or channel for carrying a signal. It can be a wire, cable, fiber optic, or wireless medium.
Telephone lines are a type of transmission medium that is used to carry voice and data signals. They are made up of copper wires that are insulated and twisted together.
Coaxial cable is a type of transmission medium that is used to carry high-frequency signals. It is made up of a central conductor that is surrounded by a layer of insulation. The insulation is then surrounded by a shield of braided copper wires.
Microwave systems are a type of transmission medium that is used to carry signals over long distances. They use microwaves, which are a type of electromagnetic radiation, to transmit signals.
I hope this helps!