d
(d) If \(\alpha \)= maximum value of base angle for which light is totally reflected form hypotenuse.

\((90^\circ - \alpha ) = C =\) minimum value of angle of incidence at hypotenuse for total internal reflection

\(\sin (90^\circ - \alpha ) = \sin C = \frac{1}{\mu }\)

\( \Rightarrow \cos \alpha = \frac{1}{\mu }\)\( \Rightarrow \alpha = {\cos ^{ - 1}}\left({\frac{1}{\mu }} \right)\)