15) D

Number of N ≡ N bonds in reactants = 1
Total energy of N ≡ N bonds in reactants = 1 * x = x
Number of H – H bonds in reactants = 3
Total energy of H – H bonds in reactants = 3 x y = 3y

Number of N – H bonds in products = 3 x 2 = 6
Total energy of N – H bonds in products = 6 x z = 6z

Since the reaction is exothermic, Energy of products must be greater than energy of reactants. 

Therefore, we get the inequality:

6z > x + 3y