This question tests your knowledge of Snell's law, Total Internal Reflection (TIR) and the fact that the refractive index of air is 1 (which should be memorised as it is in the exam). The critical angle is the angle at which TIR occurs, which can be calculated with the equation from Snell's law that relates angle of incident and refraction with the two refractive indices. The trick here is to use the angle of refraction as 90 degrees to apply the condition of TIR.