For a prism when the ray is incident at minimum deviation condition :-

The angle of prism  (angle A)  = 2 * The angle of refraction (2*30) = 60 degrees.

Refractive index of liquid = (sin((A + D)/2))/(sin(A/2)) = sin ((60+30)/2)/sin(30) = sin(45)/sin(30) =

Sqaure root of 2.

Therefore refractive index of liquid is square root of 2.