Which of the following is a resonance contributor of acrolein, $\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{CH}=\mathrm{O}$?

Resonance contributors differ only in the positions of electrons. Shifting the pi electrons of the C=C to form C=C-C and the C=O pi pair onto oxygen gives $\mathrm{CH}{}_2{}^{+}-\mathrm{CH}=\mathrm{CH}-\mathrm{O}{}^{-}$, in which the negative charge is on the more electronegative oxygen. $\mathrm{CH}{}_3-\mathrm{CH}=\mathrm{C}=\mathrm{O}$ requires a hydrogen shift and is an isomer, and the other two have the wrong charge polarity or wrong atoms.