Choose all resonance structures of ocn− with right formal charges in each. check all that apply.

Formal charge on an element in a molecule is calculated by using following formula,

Formal Charge  =  [# of Valence Electrons] - [Electrons in Lone Pairs +
                               1/2 the # of Bonding Electrons]

Example:
               Let's calculate the formal charge present on Oxygen atom in Structure 1. The oxygen atom has,
        
                                    6 Valence shell electrons

                                    2 Bond Pair Electrons

                                    6 Lone pair Electrons
So,
                        Formal Charge  =  [6] - [6+2/2]

                        Formal Charge  =  6 - 7

                        Formal Charge  =  -1

You can calculate formal charges on all atoms in all three structure. I have drawn the structures with formal charges.