Why is the equation incorrect? Mg₃ + N₂ → Mg₃N₂

ASome of the subscripts are incorrectly used.
BThe equation is not balanced.
CThe valence of the nitride ion is incorrect.
DThe valance of the magnesium ion is incorrect.
EThe coefficient of N₂ is incorrect.

Can someone explain this one?

Verified Answer :

the valency of nitride and magnesium is incorrect

The calence of nitride ion is incorrect. Nitride ion's valency is 3- (N3-)