The correct option is Diamond.
Explanation
The refractive index of a material is a dimensionless number that describes how fast light travels through the material. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium ($n = c/v$). A higher refractive index indicates a higher optical density, meaning light travels slower and bends more when entering the medium from the air.
Comparative Analysis
The approximate refractive indices for the materials listed are as follows:
- Diamond: 2.42
- Sapphire: 1.77
- Crown glass: 1.52
- Water: 1.33
Diamond has the highest refractive index among the given options. This high refractive index results in a very small critical angle (approximately $24.4^\circ$), which facilitates multiple total internal reflections within a cut diamond, contributing to its brilliance.
Key Takeaway
Diamond possesses a very high refractive index (2.42), significantly exceeding that of water (1.33) and standard glass (1.52), which is the primary physical property responsible for its optical brilliance.