Which of the following cations is more stable?

The relative stability of the contributing structures is:

Draw the resonance structures of the following compounds.
$(a)$ $CH_2=CH-Cl:$
$(b)$ $CH_2=CH-CH=CH_2$
$(c)$ $CH_2=CH-CHO$

Write the rules to decide the relative stability of different resonance structures.

Resonance in $X_2Y$ can be represented as shown in the image. The enthalpy of formation of $X_2Y$ $(X \equiv X(g) + \frac{1}{2} Y = Y(g) \rightarrow X_2Y(g))$ is $80 \ kJ \ mol^{-1}$. The magnitude of resonance energy of $X_2Y$ is $......... \ kJ \ mol^{-1}$ (nearest integer value). Given: Bond energies of $X \equiv X, X=X, Y=Y$ and $X=Y$ are $940, 410, 500$ and $602 \ kJ \ mol^{-1}$ respectively. Valence $X: 3, Y: 2$.

Which of the following cannot be represented by resonance structures?