I believe that C is correct. If you look at the moles for both the product and reactant (the coefficients in front of the compounds) the reactant has one mole (a coefficient of one) and the product has two moles (a coefficient of two). This means that one unit of the first gas/ the reactant decomposes into two units of the second gas/the product. This is all because a mole is the amount of a substance.
Hope this helped!
