If $N_A$ is Avogadro's number then number of valence electrons in $4.2\;g$ of nitride ions $(N^{3-})$ is

Question

$(a)\;4.2\;N_A \\ (b)\;2.4\;N_A \\(c)\;1.6\;N_A \\(d)\;3.2\;N_A$