For a complex reaction,the order of reaction is equal to

The molecularity of slowest step of reaction

The correct answer is option 4. The molecularity of slowest step of reaction.

In a complex reaction, multiple elementary steps may occur before the overall reaction is completed. Among these steps, one step usually proceeds at a slower rate compared to others. This slowest step is often termed the rate-determining step because it sets the pace for the overall reaction.

The order of a reaction is determined by the concentration dependence of its rate expression. For example, in a first-order reaction, the rate is directly proportional to the concentration of one reactant. In a second-order reaction, the rate is proportional to the square of the concentration of one reactant or to the product of the concentrations of two reactants.

In the context of a complex reaction, the order of the reaction is equivalent to the molecularity of the slowest (rate-determining) step. Molecularity refers to the number of molecules or species participating in an elementary reaction step. The molecularity of the slowest step dictates the concentration dependence of the overall reaction rate.

The rationale behind this is that the rate of the overall reaction cannot proceed faster than the slowest step. Hence, the rate expression of the overall reaction reflects the rate law determined by the slowest step, and thus, its molecularity dictates the overall reaction's order.

Therefore, in a complex reaction, the order of the reaction is equal to the molecularity of the slowest step of the reaction. This understanding helps in determining the kinetics of complex reactions and in predicting their behavior under different conditions.