Resonance structures of propenal are given below. Which of these resonating structures is more stable? Give reason for your answer.

As according the octet rule, the resonating structure which is having more atoms with complete octet, is stable, now looking at the structure I, we get to know that all the atoms are having complete octet, whereas in the structure II, the C-atom having positive charge is not having the complete octet, hence the structure I is more stable than structure II.