The number of molecules of the reactants taking part in a single step of the reaction tells about. :

Options

A. order of a reaction
B. molecularity of a reaction
C. fast step of the mechanism of a reaction
D. half-life of the reaction

Answer

To solve the question, we need to understand the concepts of reaction order and molecularity.

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks what the number of molecules of reactants taking part in a single step of a reaction indicates. We need to evaluate the options provided.

2. **Reviewing the Options**:
- **First Order of the Reaction**: The order of a reaction is determined experimentally and relates to the rate law, not directly to the number of molecules involved in a single step. Therefore, this option is incorrect.
- **Second Molecularity of the Reaction**: Molecularity refers to the number of reactant molecules involved in a single step of a reaction. It is defined as the sum of the number of molecules that collide to produce products. This option is likely correct.
- **Fast Step of the Mechanism of a Reaction**: The speed of a reaction step does not relate to the number of molecules involved. This option is incorrect.
- **Half-Life of the Reaction**: The half-life is a measure of the time required for the concentration of a reactant to decrease to half its initial value, which does not depend on the number of molecules involved in a single step. This option is incorrect.

3. **Conclusion**: Based on the definitions and evaluations above, the number of molecules of the reactants taking part in a single step of the reaction tells us about the **molecularity of the reaction**.

### Final Answer:
The correct option is **second: molecularity of the reaction**.

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