Answer : The overall order of reaction is, (A) zero order reaction.
Explanation :
Rate of reaction : It is defined as the rate of change in concentration of reactant or product with respect to time.
Order of reaction : It is defined as the sum of the exponents or powers to which the molar concentration in the rate law equation are raised to express the observed rate of reaction. It depends on the power of concentration of reactant.
As per question, the given rate expression is,
From this expression we conclude that there is no power of concentration of reactant. That means, it is a zero order reaction.
Hence, the rate order of reaction is, zero order reaction.