If the sender's intention is clear but the medium does not suit the receiver's needs, which principle has failed?

Correct answer: Receiver-centred medium selection

The governing principle is receiver-centred communication: effectiveness depends not only on what the sender intends, but also on whether the receiver can access, understand, and use the message. Since the intention is clear but the medium is unsuitable, receiver-centred medium selection has failed. Option C is correct; message length, sender identity, and greeting do not address the central mismatch.