A ray of yellow light (f = 5.09 * 10^14 Hz) travels

at a speed of 2.04 * 10^8 meters per second in

(1) ethyl alcohol (3) Lucite

(2) water (4) glycerol

C/v = n where c is the speed of light, v is the speed in medium and n is the refractive index of medium. Hence n = (3 x 10^8) / (2.04 x 10^8) = 1.47 that is the refractive index of glycerol. Hence answer is (4) glycerol