Final answer:
In a longitudinal wave, the rarefaction is located at the wave's trough, which is the low-pressure area where the medium's particles are furthest apart.
Step-by-step explanation:
The location of the rarefaction in a longitudinal wave is analogous to the trough of a transverse wave. Therefore, the rarefaction is the low-pressure region created in a medium when a longitudinal wave passes through it. The correct answer to the question, "Where is the rarefaction located in a longitudinal wave?" is b) At the wave's trough, as rarefaction represents the areas where the particles of the medium are furthest apart.