An echo is caused by:

**The reasoning** An **echo** is that delayed sound you hear when you shout in an empty hall or near a mountain. It happens because sound waves bounce back to you from a hard surface — just like how light bounces off a mirror. This bouncing of waves off a surface is called **reflection**. The sound travels to the wall/cliff, hits it, and returns to your ears. If the reflecting surface is far enough (at least 17 meters away), you hear the original sound and the reflected sound as two separate events — that's your echo. **Why the wrong options tempt you** - **Refraction** (A) is bending of waves when moving between different media (like sound traveling from air into water) — no bouncing back involved. - **Diffraction** (C) is when waves spread around obstacles or through openings — think sound bending around a corner. - **Polarisation** (D) applies mainly to light waves oscillating in specific directions — not relevant to sound echoes. **Quick takeaway** Echo = sound **reflecting** back to you; if it bends or spreads, that's refraction or diffraction, not an echo.