There are two conditions necessary for total internal reflection, which is when light hits the boundary between two mediums and reflects back into its original medium:
Light is about to pass from a more optically dense medium (slower) to a less optically dense medium (faster).
The angle of incidence is greater than the defined critical angle for the two mediums, which is given by:
θ = sin⁻¹(/
)
Where θ = critical angle, = refractive index of faster medium,
= refractive index of slower medium.
Choice C gives one of the above necessary conditions.