The correct answer is A.
The following tetraene upon photolysis gives a bicyclo[4.2.0]octa-2,4-diene. This is a Diels-Alder reaction, in which a diene and a dienophile react to form a six-membered ring. The diene in this case is the tetraene, and the dienophile is the ethylene molecule that is produced when the tetraene is photolyzed. The reaction proceeds through a concerted mechanism, in which the two pi bonds of the diene and the pi bond of the dienophile overlap to form the six-membered ring.
The other options are incorrect because they do not represent the product of a Diels-Alder reaction. Option B is a cyclohexene, which is not formed in a Diels-Alder reaction. Option C is a 1,3-butadiene, which is also not formed in a Diels-Alder reaction. Option D is a 1,3-cyclohexadiene, which is not formed in a Diels-Alder reaction.