The major product formed in the thermal reaction given below is 4(H)-pyran.
The reaction is a Diels-Alder reaction, which is a type of cycloaddition reaction that involves the formation of a six-membered ring between two conjugated dienes. In this case, the two dienes are the furan ring and the ethylene group. The reaction is exothermic, meaning that it releases heat.
The product of the reaction is 4(H)-pyran, which is a six-membered ring with a oxygen atom in the center. The ring is aromatic, meaning that it is stable and has a low energy.
The other options are not formed in the reaction. Option A is furan, which is the starting material. Option B is 1,3-butadiene, which is not a conjugated diene and cannot participate in the Diels-Alder reaction. Option C is 2-butene, which is not a conjugated diene and cannot participate in the Diels-Alder reaction. Option D is 1,4-pentadiene, which is a conjugated diene but it cannot react with furan because the two dienes are not in the correct orientation.