Assign formal charges to each atom in the resonance forms of n2o. which resonace form is likely to contribute most to the correct

Formal Charge is Calculated usinf following Formula,

Formal Charge  =  [# of Valence e⁻] - [# of Lone Pair e⁻ + 1/2 # of Bonding e⁻]

Lets calculate formal charge of Nitrogen in structure 3,

                              Formal Charge  =  [5] - [6 + 2/2]

                              Formal Charge  =  [5] - [6 + 1]

                              Formal Charge  =  [5] - [7]

                              Formal Charge  =  -2

You can calculate Formal charge for each atom in each resonating structure.
Also, the structure B dominates because the oxygen atom has a negative charge and being more electronegative in nature it is plausible. 

D)can be increased in a closed system