Fishing boats use sonar to locate schools of fish. What characteristic of sound is most important for this application of sonar?

We want to build a flute with two open ends. We want to flute to produce a sound of 388 Hz (fondamental). To do this, we take as a reference the sound of a tuning fork having exactly this frequency. After making the flute, we realize that the sound is not right. When listening to the flute and the tuning fork at the same time, we hear the following sound represented on the picture below. The speed of the sound in the flute is 331 ​m/s. How much to we need to reduce or increase the length of the flute for it to be in tune with the tuning fork ?

In the problem 8 of chapter 17 (attached photo), why are you using the speed of sound in air if the question specifies the submarine is in the ocean, thus, the sea water medium?

A child has an ear canal that is 1.3 cm long. At what sound frequencies in the audible range will the child have increased hearing sensitivity?