Which of the following can show resonance?

What are the characteristics of 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 $X: 3, Y: 2$.

Explain the important aspects of resonance with reference to the $CO_3^{2-}$ ion.

Identify the species having one $\pi$-bond and the maximum number of canonical forms from the following:

Which of the following resonating structures of $1$-methoxy-$1, 3$-butadiene is least stable?