Resonating structures have different

Which of the following cannot be represented by resonance structures?

The correct bond order of the $B-F$ bond in the $BF_3$ molecule is:

All bond lengths are identical in :-

Caliene,$C_7H_6$,is expected to be a fairly polar aromatic molecule. Which of the following resonance forms contributes to the greatest extent towards the real structure (resonance hybrid) of the molecule?

Write the Lewis structure of the nitrite ion,$NO_{2}^{-}$