If doubling a particular reactant concentration between two otherwise identical kinetic runs causes no change in the reaction rate, the order with respect to that reactant is zero. In other words, the reaction rate is not dependent on the concentration of that particular reactant. This is known as a zero-order reaction with respect to that reactant. The possible orders listed in the question are two, three, four, eight, sixteen, and thirty-two, but the correct answer is zero.