If there is fault in the mechanical or electrical channel, the free flow of communication will be obstructed. This obstruction is taking place at what level?

A fault in the mechanical or electrical channel that blocks the free flow of communication occurs at the transmission level.

Transmission is the physical carrying of the signal through the medium, so a defective channel disrupts it there.

The encoder level concerns the sender's framing of the message, which is intact when only the channel is faulty.

The decoder level concerns the receiver's interpretation, again untouched by a channel defect.

The feedback level concerns the return response, which is a later stage, not the carrying of the signal.

A broken wire or weak signal is a classic transmission failure.