Answer: (iii) The use of hamming codes

On a simplex link, signals travel in one direction only, so there is no return channel on which the receiver can send acknowledgements or NAKs. Any error-control scheme that depends on feedback is therefore impossible on such a link.

Hamming codes are forward error correction (FEC): the sender appends check (parity) bits, and the receiver uses them to detect and correct errors locally, entirely at the receiving end. No reverse path is needed, so Hamming coding is the only workable technique among the choices.

ARQ (ii) requires ACKs and retransmissions to flow back from receiver to sender - exactly what a simplex link cannot provide. BEC (backward error correction) and DEC (downward error correction) are not recognised error-control terms; the two real families are FEC and retransmission-based ARQ, so (i) and (iv) are distractors.