Moles of nitride ion $=\large\frac{4.2}{14}$$=0.3\;mol$
$\qquad= 0.3 \times N_A$ molecules
$\therefore$ Total valence electrons in $N^{3-}$ ion $=5+3=8$
$\therefore $ No. of valence electrons $= 8 \times 0.3 N_A$
$\qquad= 2.4 \;N_A$
Hence b is the correct answer.