The speed of sound varies significantly across different mediums due to differences in density and elasticity. In general, sound travels fastest in solids, followed by liquids, and slowest in gases. This is because molecules in solids are more tightly packed and can transmit vibrations more efficiently.

At approximately 20 °C, the speed of sound in air is around 343 meters per second, in water it is about 1,480 meters per second, in granite it ranges from 5,000 to 6,000 meters per second, and in iron, it is approximately 5,120 meters per second. Among these, the speed of sound is highest in solids like granite and iron due to their dense molecular structure.

Between granite and iron, iron typically exhibits slightly higher speeds due to its material properties. Thus, at 20 °C, the speed of sound is maximum in; option [D] Iron.