The critical angle of incidence $i_c$ for a ray incident from a denser to rarer medium, is that angle for which

the angle of refraction is 90°

The correct answer is Option (2) → the angle of refraction is 90°

Explanation:

The critical angle of incidence $i_c$ is defined as the angle of incidence in the denser medium for which the refracted ray just grazes the boundary, i.e. the angle of refraction $r = 90^\circ$.