[OfficialAnu]

Are they right? I checked my answer with a software the generates resonance forms and it produced all forms except B and C.

[mjc123]

Though B and C are technically possible, they have 3 separate charges. They will therefore be much less stable than the forms with only 1 charge, and contribute negligibly to the resonance. Normally you would not include such structures when asked for the resonance forms of a molecule/ion.