Traveling Sound Waves

For an open organ pipe, the fundamental frequency is the first harmonic, while for a closed organ pipe, the first overtone is the third harmonic. The frequencies are matched using the formula for harmonics in pipes, and the length of each pipe is calculated.

An echo is heard when sound waves are reflected off a surface and return to the source. In this case, the engineer hears the echo of the whistle because the sound waves from the whistle are reflected by a surface (e.g., a building or a hill), and they return to the engineer. This is an example of reflection of sound waves.

Hence, the correct answer is (a).