The correct answer is C. Channel.
A channel is a path linking two hardware or telecommunication units down which data and control signals can be transmitted. It is a medium through which information is transmitted. Channels can be physical, such as a wire or cable, or they can be logical, such as a network connection.
Modulation is the process of changing one or more properties of a carrier signal with a message signal. The carrier signal is a high-frequency signal that is used to transmit the message signal. The message signal is the information that is to be transmitted.
Demodulation is the process of recovering the message signal from a modulated carrier signal. The demodulator is a device that performs the demodulation process.
Signalling is the process of sending and receiving information using signals. Signals can be electrical, mechanical, or optical.
None of the above is not the correct answer because it does not describe a path linking two hardware or telecommunication units down which data and control signals can be transmitted.