Which one of the following statements is not correct?

Option 3 is not correct. The speed of sound in air rises with increasing temperature — for an ideal gas the acoustic speed depends on temperature (not pressure) as shown by its dependence on T in the standard relation, so statements 1 and 2 are correct [2]. Humidity does affect the speed of sound, but it does not make it decrease; adding water vapour (which lowers the mean molecular weight of the air mixture) slightly increases the sound speed — the humidity effect is small compared with temperature effects, but it is in the increasing, not decreasing, direction. Linear acoustic theory (small-amplitude waves) implies speed is independent of amplitude and frequency, so statement 4 is correct [2].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > 20.2. Structure of The Earth's Atmosphere > p. 274
2. [2] https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/sound.html