c
For total internal reflection, \(\sin i > \sin C\) where, \(i\,=\) angle of incidence, \(C\,=\) critical angle But, \(\sin C = \frac{1}{\mu }\)     \(\therefore \quad \sin i > \frac{1}{\mu }\) or \(\mu  > \frac{1}{{\sin i}}\)

\(\mu  > \frac{1}{{\sin {{45}^\circ }}}\)             \(\left(i=45^{\circ}(\text { Given })\right)\)

\(\mu  > \sqrt 2 \)