Question

a mosquito is said to produce sound at high pitch

Answer 1

Pitch is that characteristics of sound by which we can distinguish between different sounds of same loudness.

-->Pitch of sound depends on the frequency of  vibration. 
--> Pitch of sound is directly proportional to  its frequency.
--->If frequency of vibration is high then sound produced by it has a high pitch..

So, In case of Mosquito, The frequency of vibration of its wings is more, so it has more pitch.