How is energy from flowing water different from iron, petroleum or coal ?

The energy produced from flow of water or "Hydro-power" is different to energy produced from iron, petroleum and coal as it is a clean and renewable form of energy. Hydro-power is derived from the kinetic energy of the natural flow of water like in the case of a water mill whereas the other forms listed require combustion of the fuel source that produces harmful gases leaving a negative impact on the environment. Also, hydro-power is one of the cheapest forms of energy production.  
However, it should be noted that large-scale production of electrical energy from flowing water known as "Hydroelectric Power" uses man-made dams to store water in lakes as a source of potential energy. These dams can also have a negative impact on the environment as the natural flow of water is restricted. Some scientists also suggest that the construction of a dam leads to an increase in the number of earthquakes in and around the area of construction.

The right answer is c.sandstone.