Explanation: A crystal band-pass filter for single-sideband (SSB) phone needs to match the essential frequency range of human speech for good intelligibility without wasting spectrum. Human speech, for effective communication, primarily occupies frequencies from approximately 300 Hz to 3000 Hz. Since SSB transmits only one sideband, the required bandwidth for the filter is roughly the bandwidth of the audio signal itself. A 2.1 KHz bandwidth (e.g., passing frequencies from 300 Hz to 2400 Hz) provides excellent intelligibility for voice communication while being spectrally efficient. Options like 5 KHz or 15 KHz are excessively wide for SSB, leading to inefficient spectrum use and increased noise pickup. Conversely, a 500 Hz bandwidth is too narrow for voice, severely degrading intelligibility by cutting off significant portions of the speech frequencies, though it's suitable for Morse code (CW) reception. Therefore, 2.1 KHz is an ideal compromise for SSB voice.