Geo-Scientist Prelims 2020 Chemistry Q18: How many resonating structures are possible for NO₃⁻ ion?

Answer: (c) 6

NO₃⁻ (24 valence e⁻): N forms one N=O and two N–O⁻. The double bond can sit on any of the 3 O atoms → 3 equivalent major contributors, each N–O bond order 4/3. UPSC's key counts 6, i.e. it also counts the minor charge-separated forms → (c).