Explanation: A 2.4 kHz receiver selectivity is optimum for SSB (Single Sideband) voice because it closely matches the typical bandwidth of a human voice signal. SSB transmissions efficiently convey voice information within approximately 2.4 to 2.7 kHz, which includes the critical audio frequencies for clear communication. A 2.4 kHz filter effectively passes this full voice range while rejecting noise and interference from signals outside this bandwidth, maximizing the signal-to-noise ratio and audio fidelity. For CW (Morse code), 2.4 kHz is far too wide; CW signals are very narrow, typically 50-500 Hz. Double-sideband AM voice signals require about 6 kHz of bandwidth (twice the audio bandwidth), so a 2.4 kHz filter would severely clip and distort them. FSK RTTY signals are also relatively narrow, ranging from a few hundred Hz to around 1 kHz, making 2.4 kHz generally too wide.