How many resonance forms can be written for the nitrate ion,$(NO_3^-)$?

Which of the following resonating structures of $N_2O$ is the most contributing?

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

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

Delocalised molecular orbitals are found 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?