Cyclohex-3-ene-1-carbaldehyde is best prepared by a Diels-Alder reaction between:

Disconnecting the ring at the bonds next to the double bond (C3=C4) gives a 4-carbon diene and a 2-carbon dienophile bearing the $\mathrm{CHO}$ group. These are buta-1,3-diene and $\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CHO}$. Propanal has no C=C, and isoprene would add a methyl group.