Solar panels contain a very expensive element, i.e. silicon. This element is able to convert the solar energy into electric energy. The cost of the solar panel gets expensive because of this rare element. It is found in limited areas of the world. That’s why option D is correct. Coal is not used in solar panel, so option B is incorrect. Neither is wind energy, so option A is also incorrect. Water is also not used for exploiting solar energy, which makes option C a wrong answer.
