For incident ray C, the angle of refraction is 90°. The refracted ray C has the smallest amount of energy of any refracted ray. Thus, it would be an extremely "dim" light ray. What is the angle of incidence for ray C called?

When a ray of light travels into rarer medium from the denser medium and the angle of refraction is 90° so the angle of incidence in the denser medium is called critical angle for that pair of media.

Here, the angle of refraction is 90°, so the angle of incidence C is called the angle of incidence which is equal to the critical angle.