Answer: cycloaddition of 1,3-butadiene and allyl anion is thermally allowed.
Explanation:
Using the frontier molecular orbital approach, we consider if the symmetry of the LUMO of the diene (1,3-cyclobutadiene) and the HOMO of the dienophile allyl anion), match each other since the diels alder reaction is a symmetry allowed process. The symmetries of these frontier molecular orbitals match each other hence the reaction is thermally allowed. It should be notes that 4+2 cycloadditions are thermally allowed although they are slow.
