Apanpipe is made of five pipes. the longest pipe is 25 centimeters long
and the shortest is 5 centimeters long. which of these pipe produces the
highest-frequency sound and why?

The pipe that produces the highest-frequency sound is the 5 cm long pipe. 

According to the Fundamental Principle, the length of the tube of the pipe and its frequency is inversely proportional. 

This means that the longer the tube, the lower the frequency and vice versa. Therefore, between the 25cm long pipe and the 5cm long pipe, the shorter pipe produces the highest-frequency sound.