Question 21B2

ARQ (Automatic Repeat reQuest) systems require a return channel for the receiving station to acknowledge reception or request retransmission of corrupted data. This inherent interactivity makes ARQ a **two-way communication** method. FEC (Forward Error Correction) involves adding redundant data to a transmission, allowing the receiver to detect and correct errors without needing to request retransmissions from the sender. As no return path is required, FEC is efficient for **one-way communications to all stations** (broadcasts). SFEC (Selective Forward Error Correction) is a more specialized form of FEC where the error correction might be applied selectively or targeted to a specific receiver or a small, defined group, rather than a universal broadcast. While still a one-way method, its "selective" nature distinguishes it, aligning with **one-way communications to a single station**. Therefore, the sequence ARQ, FEC, SFEC corresponds to Two-way communications, one-way communications to all stations, one-way communications to a single, making option B correct. Options A, C, and D are incorrect because they misalign the fundamental nature (two-way vs. one-way) or the scope (all stations vs. single station) for one or more of the protocols.