Assertion (A) is correct: Communication channels serve as the medium through which a message travels from sender to receiver, encompassing both physical mediums (e.g., written letters, in-person conversations) and non-physical mediums (e.g., emails, phone calls). However, Reason (R) is incorrect because the stated dimensions of a channel—mode of encoding and decoding messages, message-vehicles, and vehicle carriers—are not a standard or accurate framework for describing communication channels. Typically, channels are categorized by their nature (e.g., verbal, non-verbal, written, digital) or characteristics like speed, reach, and feedback capability, not the terms provided in (R). Thus, while (A) accurately describes the role of channels, (R) does not provide a correct or relevant explanation.