The answer is: C. 3 : 2.
Balanced chemical reaction: 3H₂ + N₂ → 2NH₃.
From chemical reaction: n(H₂) : n(NH₃) = 3 : 2.
Coefficients with the lowest ratio indicate the relative amounts of substances in a reaction.
Coefficient in fron of hydrogen is 3 and coefficient in fron ammonia is 2.
Coefficients are important to prove the law of conservation of mass.