Answer :

Sound waves move the most quickly through materials that are more dense. The speed of sound is generally faster in solids compared to liquids and gases because the particles are closer together, allowing the sound waves to travel more efficiently.

1. Among the options given:
- Iron is a solid and has a high density, making sound waves travel fastest through it.
- Water is a liquid, so sound waves travel slower through it compared to solids.
- Air is a gas and has the lowest density among the options, resulting in slower sound wave transmission compared to solids and liquids.

Therefore, the correct answer to the question is:
Sound waves move the most quickly through Iron.