AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
(D) Air
Light travels fastest in air among the given options, as air has the lowest refractive index (1.0003), meaning the speed of light in air is closest to its speed in vacuum ($3 \times 10^8$ m s$^{-1}$).
Source: Chapter 9, Section 9.3.2 (Table 9.3)
Higher refractive index = lower speed of light. From Table 9.3: Air (1.0003) < Water (1.33) < Crown glass (1.52) < Diamond (2.42). So light is slowest in diamond and fastest in air among these four options. Remember: speed and refractive index are inversely related.