Correct option is D
Indole is an important heterocyclic system because it is built into proteins in the form of the amino acid tryptophan, because it is the basis of important drugs such as indomethacin, and because it provides the skeleton of the indole alkaloids.
The Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. More specifically, it is classified as a thermally allowed [4+2] cycloaddition reaction.
