Resonance forms are forms of the same molecules where electrons have changed their location. All resonance structures must have the same number of electrons. Let's first count the number of valence electrons in N3-. Nitrogen is in group 5 on the Periodic table and therefore has 5 valence electrons. Minus charge means we need to add one extra electron. We get, 5*3+1 = 16.
Now, lets see which of the answer choices does not have 16 electrons. One bond is two electrons. Choice A has 12 electrons in lone pairs + 6 electrons in bonds (3 bonds). We get 18 electron. Choices B, C and D have 16 electrons. Choice A is the correct answer.
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