Correct answer: (c) 3

Explanation:

Calculating the oxidation state of nitrogen in given molecules;

Oxidation state of N in NH₃ is

x + 3 × (+1) = 0 or x = -3

Oxidation state on N in HNO₃ is

1 + x + 3 × (-2) = 0 or x = + 5

Oxidation state of N in NaN3 is

+ 1 + 3x = 0 or x = \(-\frac{1}{3}\)

Oxidation state of N in Mg₃N₂ is

3 × 2 + 2x = 0 or x = -3

Thus 3 molecules (i.e. NH₃, NaN₃ and Mg₃N₂ have nitrogen in negative oxidation states.