Concept:An echo is heard when sound waves bounce back from a large, hard surface. This is called reflection of sound.Explanation:Sound travels in straight lines. When it hits a hard surface, it reflects like light. The reflected sound reaches the listener after a short delay, creating an echo.Other options are incorrect: refraction bends sound through different media, diffraction spreads sound around obstacles, and resonance amplifies sound at natural frequencies. None of these produce a delayed repetition of sound.Answer:B. Reflection of sound waves