Answer:
A.It is a reaction involving pi-electrons.
D.It is *not* a reaction favored by entropy considerations.
Explanation:
The Diels-Alder reaction is a [4+2]- cycloadition that involoves pi-electrons (4 from the diene and 2 from the dienophile).
The reaction is favored by the formation of new sigma bond, since they are more stable than pi-bonds. This reaction is not favored by entropy because there are 2 reactants and one product and the order increases.
The reaction happens in one step.
