Resonance is caused by the delocalization of which of the following?

Delocalised molecular orbitals are found in

The increasing order of stability of the resonance structures is:

$A$ molecule may be represented by three structures having energies $E_1$,$E_2$ and $E_3$,respectively. The energies of these structures follow the order $E_3 < E_2 < E_1$. If the experimental bond energy of the molecule is $E_0$,the resonance energy is

Resonance structures of propenal are given below. Which of these resonating structures is more stable? Give reason for your answer.
$CH_2=CH-CH=O$ $(I)$ $\leftrightarrow$ $\stackrel{+}{CH_2}-CH=CH-\stackrel{-}{O}$ $(II)$

The stability order of the following resonance structures is:
$I. CH_2=CH-CH=O$
$II. CH_2^+-CH=CH-O^-$
$III. CH_2^--CH=CH-O^+$