Final answer:
A hard, flat piece of wood would best reflect sound waves and create an echo, as it provides the optimal surface for coherent reflection of sound. Soft or bumpy surfaces are not ideal for this purpose as they absorb or scatter the sound.
Step-by-step explanation:
To create an echo, a sound wave needs a surface that is both hard and flat to effectively reflect the sound waves back to the listener. The correct option among those provided would be (b) a hard, flat piece of wood. This type of surface will reflect sound waves without absorbing them, unlike soft materials, and the flatness ensures that the reflection is coherent, therefore when it reaches the listener's ear it produces an audible echo. In contrast, a bumpy surface would scatter the reflected sound waves and a soft surface would absorb them, thus a hard, flat piece of wood is the best reflector of sound to create an echo.
When we consider the interference of sound waves as they move through a room, we know that the sound is louder where constructive interference occurs and softer where destructive interference occurs. An empty school hall with large, flat surfaces is an ideal environment for hearing reflected sounds, as the sound waves can reflect coherently, which is necessary for an echo to be heard.