Which structure has delocalised $\pi$-electrons?

$CH_2 = O \leftrightarrow \overset{\oplus}{C}H_2 - \overset{\ominus}{O} \leftrightarrow \overset{\ominus}{C}H_2 - \overset{\oplus}{O}$
Which of these structures is practically not a valid canonical structure for formaldehyde?

Provide the Lewis dot resonance structures of nitrogen oxides and give information about their shapes and bond parameters.

Explain with a suitable example that the resonance structures of a molecule do not represent its real structure and are hypothetical.

The difference in energy between the actual structure and the lowest energy resonance structure for the given compound is

The increasing order of stability of the resonance structures is: