A ray of light from a denser medium strikes a rarer nedium at an angle of incidence $i$. The reflected and refracted rays make an angle $90^{\circ}$ with each other. The angle of reflection and angle of refraction are $r$ and $r$ ' respectively. The critical angle is

(a) $\sin ^{-1}(\tan r)$

(b) $\cos ^{-1}(\tan r)$

(c) $\sin ^{-1}\left(\tan r^{\prime}\right)$

(d) $\sec ^{-1}(\tan r)$