The correct answer is option 2 - multiple reflection of sound. Reverberation is a phenomenon that occurs when sound waves are reflected multiple times before dying out. When sound waves encounter a surface, they bounce off and reflect in different directions. These reflected waves can then collide with other surfaces and reflect again. The process of multiple reflections creates a prolongation of sound that can be heard even after the original sound source has stopped producing sound.

Option 1, multiple refraction of sound, is incorrect because refraction refers to the bending of sound waves when they pass through different mediums, such as air to water or vice versa. Reverberation is not caused by refraction.

Option 3, single refraction of sound, is also incorrect because as explained above, refraction is not the cause of reverberation.

Option 4, single reflection of sound, is incorrect because reverberation is a result of multiple reflections, not just a single reflection of sound waves.

To summarize, reverberation is associated with multiple reflections of sound waves, as they bounce off surfaces and continue to collide and reflect before eventually dying out.