The bandwidth sufficient for commercial telephonic communication:
300 Hz
700 Hz
2800 Hz
1400 Hz
For speech signals (as in telephone) frequency range 300 Hz- 3100 Hz is adequate i.e., a speech signal requires a bandwidth of 2800 Hz.
A source emits sound of frequency 600 Hz inside water. The frequency heard in air will be:
(V = 1500 m/s in water and V = 300 m/s in air).