What is Wi-Fi?

Answer: A. On a wireless channel it is hard to detect a collision by receiving other stations' weak signals while transmitting, so stations wait before sending to avoid collisions as far as possible

CSMA/CD is used on shared wired media such as old hub-based Ethernet: a station that detects a collision while sending stops and resends after a random time. On a wireless channel, a station's own signal is so strong that it is hard to receive a distant station's weak signal at the same time, and collisions between stations that cannot hear each other (hidden stations) are invisible to the sender. So with CSMA/CA, a station confirms the channel is idle and then waits a fixed time plus a random backoff before sending. The receiver returns an ACK when it receives the frame correctly, and if no ACK arrives the sender assumes a collision or similar and retransmits. In today's wired Ethernet, connected full duplex to switches, collisions do not occur and CSMA/CD is effectively unused.

Answer: A. The 2.4 GHz band reaches farther even with obstacles, but it is shared with microwave ovens, Bluetooth and others, so it gets congested easily and has few non-overlapping channels

In general, the higher the frequency, the more easily radio waves are weakened by walls and other obstacles and the shorter their range. The 2.4 GHz band reaches well, but as a licence-free ISM band it is used by many devices such as microwave ovens and Bluetooth, so it suffers interference, and only about three or four 20 MHz channels do not overlap one another. The 5 GHz band has more channels and makes it easy to bond them into wider channels for higher speed, but it is weaker against obstacles, and some channels require the DFS procedure to avoid interfering with weather radar and the like. In the 2020s, Wi-Fi 6E and Wi-Fi 7 also opened up the 6 GHz band (the usable range differs by country and region).