A pure substance is a chemical substance that has a constant chemical composition. An element, molecule, and compound are pure substances but not a solution. Thus, option a is correct.
A pure substance is a matter that has been constituted of constant and definite composition with distinguishable chemical properties. The substances are non-transformable and cannot be broken down.
An element is a pure substance that cannot be broken into new substances by chemical and physical methods. A compound can be a pure substance too when the fixed ratios of the elements involved combine chemically.
A solution is not a pure substance as they are homogeneous mixtures that are composed of elements in unequal ratios and can be separated through chemical and physical means like evaporation, filtration, etc.
Therefore, option a. solutions are not pure substances.
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