You are given kerosene, turpentine and water. In which of these the light travels fastest?

The light travels fastest in that medium which has the lowest refractive index i.e. lowest optical density. In the medium having lowest refractive index, the speed of light rays is high. From the three materials given, kerosene, turpentine and water, water has the lowest refractive index (of 1.33), so light travels fastest in water.