The correct answer is A.
The Diels-Alder reaction is a pericyclic reaction that involves the formation of a six-membered ring between a diene and a dienophile. The diene must have two double bonds that are conjugated, and the dienophile must have a double bond or triple bond that is capable of forming a six-membered ring.
In option A, the diene is 1,3-butadiene, which has two double bonds that are conjugated. The dienophile is maleic anhydride, which has a double bond that is capable of forming a six-membered ring. Therefore, option A is the only option that satisfies the requirements for the Diels-Alder reaction.
In option B, the diene is 2-butene, which does not have two double bonds that are conjugated. Therefore, option B is not a valid diene for the Diels-Alder reaction.
In option C, the diene is 1-butene, which does not have two double bonds that are conjugated. Therefore, option C is not a valid diene for the Diels-Alder reaction.
In option D, the diene is ethylene, which does not have two double bonds that are conjugated. Therefore, option D is not a valid diene for the Diels-Alder reaction.