Imagine that you're standing in a large room when a loud noise is made. You begin hearing a series of echoes. Which characteristic of sound best explains what just happened?

The characteristic of sound that would best explain the series of echoes you are hearing would be reverberation. It is created when a sound wave is being reflected to the surfaces causing it to have a many reflection building up and decaying as these waves are being absorbed bu the surfaces in the area which could be tables, chairs, people or the air. Reverberation is the group of the reflected sounds in an enclosed area.