The correct answer is B. An underground lake
Explanation:
Sound moves more slowly in liquids or gases than in solids, in which particles are together. Additionally, the speed of sound is measured in meters per second or m/s. In this context, if the speed of a sound wave reduced from 4,000 m/s to 1,500 m/s while traveling through the surface of Earth, which is solid, it is likely the cause is a change in the medium the wave is going through.
Moreover, the average speed of sound in air is below 400 m/s, this suggests the sound was not traveling through an open cavity. However, the speed in liquids such as water is around 1480 m/s, this is quite similar to the second speed registered. According to this, the sound wave slowed because it was traveling through a liquid medium; therefore, the answer is an underground lake.
