Explanation

(c)
The oxidation state of N in ${NH}_{3}$ is
$x + 3( + 1) = \ 0$
$x =$ $- 3$
The oxidation state of N in $HNO_{3}\$is
$1 + x + 3( - 2) = 0$
$x = 5$
The oxidation state in N in ${NaN}_{3}$ is
$+ 1 + 3x = 0$
$x = - 1/3$
The oxidation state of N in ${Mg}_{3}N_{2}$ is
$3(2) + 2x = 0$
$6 + 2x = 0$
$x = \ - 3$
Hence, three molecules ($i.e.,{NH}_{3},\ {NaN}_{3},\ {Mg}_{3}N_{2})\$have negative oxidation state.