In a closed hall, a sound continues to be heard briefly because it reflects repeatedly from the walls, ceiling, and floor. What is this effect known as?

The correct option is Reverberation.

Step-by-Step Explanation:

1. Definition of Reverberation:
When a sound is produced in an enclosed space such as a closed hall or room, the sound waves travel outward and strike the surrounding surfaces, including the walls, ceiling, and floor. Instead of disappearing immediately, the sound wave undergoes multiple, repeated reflections off these hard boundaries. The continuous overlay of these reflected sound waves causes the sound to linger or persist for a short duration even after the original sound source has stopped vibrating. This phenomenon of persistent sound caused by continuous multiple reflections is known as reverberation.

2. Why the Other Options are Incorrect:
• Refraction: Refraction refers to the bending of a wave when it passes from one medium into another of different density or sound speed.
• Diffraction: Diffraction is the bending or spreading of sound waves around corners or through small openings.
• Rarefaction: A rarefaction is the low-pressure region in a longitudinal wave where the particles of the medium are spread farthest apart.

Hence, the effect described in the question is correctly identified as reverberation.