Answer
Box (d) best represents the reactants, and box (c) best represents the product.
Work Step by Step
Start by counting and identifying the molecules in each box.
(a) $6N_2O$
(b) $6NO$
(c) $6NO_2$
(d) $6NO$ and $3O_2$
Now, compare them with the reactants and the products of this reaction:
Box (d) represents the reactants ($2NO + O_2$)
Box (c) represents the product ($2NO_2$)
