Explanation:

- Option 1: Air

- Air has a refractive index of approximately 1.0003.

- It's very close to a vacuum, so light travels through air almost as fast as it does in a vacuum.

- Option 2: Diamond

- Diamonds have a high refractive index of about 2.42.

- This property gives diamonds their brilliant sparkle and reflects light internally.

- Option 3: Crown Glass

- Crown glass typically has a refractive index around 1.52.

- It's commonly used in lenses and optics due to its clarity and optical properties.

- Option 4: Water

- Water has a refractive index of about 1.33.

- This is why objects appear bent or distorted underwater.