At the glass air interface on the hypotenuse of the prism reflection takes place, $\theta$ must be greater than the critical angle

$\theta > \theta_c$

$\sin \theta < \sin \theta_c$

$\large\frac{1}{\sqrt 2 } > \frac{1}{\mu g}$

$\mu g > \sqrt 2$

Hence d is the correct answer.