n₂          n_air 
sin theta_critical = ----- =   --------
                             n₁         n_liquid   

    
                       n_air                        1
n_liquid  =  ---------------------- = ----------- = 1.29
                  sin theta_critical       sin 51^o   

    
Observe that total internal reflection only occurs when the wave is passing from a medium with a higher index of refraction (lower speed) to a medium with a lower index of refraction (higher speed).  This is true because the Sine cannot be greater than 1.

The Correct Answer is