a b c d

b

$\text{For ray to emerge parallel to water surface angle of incidence should be critical angle }$ $\Rightarrow r = C = sin^{-1}\frac{1}{\mu_w} = sin^{-1}\frac{3}{4}$ $\text{Apply snells law of water - glass interface }$ $\Rightarrow \mu_g sin i = \mu_w sinr =1$ $\Rightarrow \mu_g = \frac{1}{sini}$