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

The correct option is: The molecularity of the slowest step of the reaction.

Here is the step-by-step educational explanation of why this option is correct:

1. Understanding Complex Reactions:
A complex reaction is a chemical reaction that does not occur in a single step. Instead, it proceeds through a sequence of two or more elementary (single-step) reactions. This sequence is known as the reaction mechanism.

2. The Rate-Determining Step:
In any multi-step (complex) reaction, the individual steps proceed at different rates. The overall rate of the chemical reaction is limited by the slowest step in the mechanism. Therefore, the slowest step is called the rate-determining step (RDS) of the reaction.

3. Relating Molecularity and Order:
For any elementary reaction (including the slowest step of a complex mechanism), the molecularity and the order of the reaction are identical. Molecularity refers to the number of reactant molecules colliding simultaneously to bring about the chemical change in that elementary step.
Since the rate-determining step dictates the rate law of the overall reaction, the overall order of the complex reaction is equal to the order of this slowest step. Consequently, the overall order is also equal to the molecularity of the slowest step.