One mole of magnesium nitride on reaction with excess of water gives :

Step 1: Formula of magnesium nitride.

The formula for magnesium nitride will be ${\mathrm{Mg}}_3{\mathrm{N}}_2$.

Step 2: Reaction of magnesium nitride with an excess of water${\mathrm{Mg}}_3{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{Mg}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_3$

Hence one mole of magnesium nitride on reaction with excess water gives two moles of ammonia.

Final Step: So the correct answer is B.