The correct answer is (d), modem. 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 for carrying a signal. It can be a wire, cable, fiber optic, or wireless link. The type of transmission medium used depends on the distance to be covered, the amount of data to be transmitted, and the required bandwidth.
(a) Telephone line is a type of transmission medium that uses copper wires to carry signals. It is a common type of transmission medium for voice and data communication.
(b) Coaxial cable is a type of transmission medium that uses a central conductor surrounded by a shield. It is a high-speed transmission medium that is often used for cable television and internet access.
(c) Microwave system is a type of transmission medium that uses microwaves to carry signals. It is a high-speed transmission medium that is often used for long-distance communication.