The critical angle for total internal reflection at a glass-air interface is $42^\circ$. The refractive index of the glass is approximately:

For total internal reflection at a denser-to-rarer interface, $\mu = \dfrac{1}{\sin C}$. With $C = 42^\circ$, $\sin 42^\circ \approx 0.669$, so $\mu = 1/0.669 \approx 1.49$, which matches typical crown glass. **Option A is water and option D is diamond.**