The diene shown reacts with propenal ($\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CHO}$) in a Diels-Alder reaction. Which structure is the major regioisomer, the so-called 'para' (1,4) adduct?

The compound shown is isoprene. The 2-methyl group makes C1 of the diene the most nucleophilic end, and it bonds to the electrophilic beta carbon of propenal. The aldehyde-bearing carbon then lies 1,4 to the methyl-bearing alkene carbon, giving 4-methylcyclohex-3-ene-1-carbaldehyde. The 1,3 and 1,2 products are the minor 'meta' and 'ortho' isomers, and the last option has the double bond in the wrong place.