Answer
both (a) and (c)
Work Step by Step
When light travels from denser to lighter medium $(n_{1}>n_{2})$, for a certain angle of incidence called the critical angle , the angle of refraction is $90^{\circ}$ , and the refracted ray is directed along the boundary between the media.
If the angle of incidence is greater than the critical angle $\theta_{1}>\theta_{c}$ , the light isn’t refracted at all, but is internally reflected. So, both (a) and (c) are true.
