$(a)\;2.62 \times 10^{8}\;m/s \\ (b)\;1.26 \times 10^8\;m/s \\ (c)\;4.21 \times 10 ^8\;m/s \\ (d)\;1.73 \times 10^8\;m/s$

$\mu= \large\frac{Velocity \;of\;light\;in\;air}{Velocity\;of\;light\;in\;medium}$

$\qquad= \large\frac{ \sin i}{\sin r}$

$\large\frac{c}{v}= \frac{\sin 60}{\sin 30}$

$v= \large\frac{c \times\sin 30}{\sin 60}$

$\quad= \large\frac{3 \times 10^8 \times \Large\frac{1}{2}}{\Large\frac{\sqrt 3}{2}}$

$\quad= \large\frac{3 \times 10^8}{\sqrt 3}$

$\quad=1.73 \times 10^8 m/s$

Hence D is the correct answer.

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