The theory of evolution was proposed by Darwin.
A should be the products and D should be the reactants. So D should be the answer.
The given blank can be filled with isomers.
The isomers in chemistry refers to the molecules or ions with similar formulas, but different compositions. The isomers refer to the molecules that exhibit the same chemical formula, however, distinct three-dimensional shapes. Though isomers do not always share identical properties. The two prime forms of isomerism are stereoisomerism or spatial isomerism and structural isomerism or constitutional isomerism.