SOLUTION E3.4
The alkene (2-butene) has no conjugation, so largest HOMO-LUMO gab and smallest lmax. The alkene anion is part of a conjugated system with three π-MOs, making the HOMO-LUMO gap smaller than in the isolated alkene (larger lmax). The conjugated aldehyde has the largest lmax due to the largest amount of conjugation in the system (four π-MOs).
