A ray of light passes from a medium L to a medium K. No refraction of light occurs if the ray of light hits the boundary of these two media at the angle of incidence equal to ...............

The correct option is B ${90}^{\circ }$
If a light is an incident at an angle of ${90}^o$ it will go straight again making an angle of ${90}^o$ with the normal it doesn't depend on medium.
so the angle of incidence is ${90}^o$
A because if an angle of incidence is 90°, it means the light is propagating along the interface separating the two media. In this case, the light does not enter the second medium and so we can not talk about the angle of refraction. The angle of refraction can also be regarded as 90°.