If $N_A$ is the Avogadro’s number, then the number of valence electrons in $4.2\text{g}$ of nitride ions $\left(N^{3-}\right)$ is:

The correct option is B $2.4N_A$
Moles of nitride ion $\Rightarrow \frac{givenmass}{molarmass}$
$\Rightarrow \frac{4.2}{14}=0.3N_A$ nitride ions.
The number of valence electrons in an nitride ion $=8(5+3\text{due to the charge})$
The number of valence electrons in $4.2\text{g}$ of nitride ions$\Rightarrow 8\times 0.3N_A=2.4N_A$