The correct option is D.
In chemistry, a pure substance refers to a matter that possesses definite and constant composition with unique chemical properties. Another word for a pure substance is chemical substance. A pure substance typically contains only one type of atom or only one type of molecule. Examples of pure substances are water, sodium chloride, gold, silver, etc. In the question given above, we are told that the unknown element can only be broken down by chemical mean, this shows that the unknown compound is a chemical compound.
