Total internal reflection occurs when:

Correct answer: d angle of incidence > critical angle

When light goes from denser to rarer and the angle of incidence exceeds the critical angle, refraction is no longer possible; all the light returns into the denser medium.

Why the other options are wrong

• a angle of refraction < angle of incidence: The angle of refraction is smaller than the angle of incidence when light goes from rarer to denser, and then refraction always happens. From denser to rarer the angle of refraction is the larger one.
• b angle of incidence > angle of reflection: By the law of reflection the angle of reflection always equals the angle of incidence, so this condition never occurs. The condition for total reflection compares the angle of incidence with the critical angle.
• c critical angle > angle of refraction: In total reflection there is no refracted ray, so comparing with the angle of refraction means nothing. The condition is that the angle of incidence exceeds the critical angle.