Us own words,
question:

explain the law of conservation of energy. give a specific example using kinetic and potential energy that shows how energy is conserved.

energy can neither be created nor be destroyed but it can only change  from one form to another or transfer from one to another object this is the simplest way of understanding the law of conservation of energy. In simple line its the transfer of energy only.

so now the potential energy is the energy that is in stored form while kinetic energy is that energy which an object contain due to any particular motion.

like a ball is thrown in air so the energy through which it goes upward is kinetic energy and for downward movement of ball it need another energy which is potential energy(in the stored form) and by using this energy it goes downward.

answer; i believe that the correct answer is (a) potassium(k)