The number of resonance structures for $N$ is

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 structures represents the correct resonance structures of boron trifluoride $(BF_3)$?

The correct order of $C-O$ bond length is:

The resonance hybrid of the nitrate ion $(NO_3^-)$ is represented as:

In the anion $HCOO^-$,the two carbon-oxygen bonds are found to be of equal length. What is the reason for it?