# The number of outer electron in $N_3^-$ are

$\begin{array}{1 1}(a)\;\text{16 electrons}&(b)\;\text{22 electrons}\\(c)\;\text{21 electrons}&(d)\;\text{15 electrons}\end{array}$

$N_3^-$ (Azide ion) has 16 outer electrons.
(Five from each N and one from the charge on the ion)
Hence (a) is the correct answer.