Rate of a reaction is defined as the change in the concentration of the reactant or product in unit time.

Rate= =

Where R is the change in concentration of a reactant in a time interval t and P is the change in concentration of a product in a time interval t.

For such type of a reaction, the rate is given by dividing the rate of appearance or disappearance of product and reactant respectively by their stoichiometric coefficients hence the rate of reaction become,

Rate =

Rate=

Thus option (iii) is correct.