Correct Option: C

The speed of light with the rise in the temperature of the medium remains unaltered because speed of light doesn’t depend on temperature. It does depend, however, on the refractive index of the substance it is travelling through. The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time. The speed at which light propagates through transparent materials, such as glass or air, is less than c.