The correct answer is D. Modem.
A modem is a device that converts computer output into a form that can be transmitted over a telephone line. It does this by converting digital signals into analog signals, which can then be transmitted over the telephone line. At the other end of the line, another modem converts the analog signals back into digital signals, which can then be read by the computer.
A teleport is a facility that provides access to the Internet and other telecommunications services via satellite. It does this by receiving and transmitting signals from satellites.
A multiplexer is a device that combines multiple signals into a single signal. This can be done for the purpose of transmitting multiple signals over a single channel, or for the purpose of storing multiple signals in a single memory location.
A concentrator is a device that combines multiple signals into a single signal for the purpose of transmitting them over a single channel. This can be done to reduce the number of channels that are required, or to increase the capacity of a single channel.