Assertion (A) The enthalpy of reaction remains constant in the presence of a catalyst.
Reason (R) A catalyst participating in the reaction froms different activated complex and lowers down the activation energy but the difference in energy of reactant and product remains the same.

Options

A. Both assertion and reason are correct and the reason in correct explanation of assertion.
B. Both assertaion and reason are correct, but the reason does not explain essertion
C. Assertion is correct but reason is incorrect
D. Both assertion and reason are incrrect.

Explanation

To solve the question regarding the assertion and reason about catalysts and enthalpy of reaction, we can break it down step-by-step:

### Step 1: Understand the Assertion
The assertion states that "The enthalpy of reaction remains constant in the presence of a catalyst."
- **Explanation**: This means that adding a catalyst does not change the overall energy difference between the reactants and products of a chemical reaction.

### Step 2: Understand the Reason
The reason provided is that "A catalyst participating in the reaction forms different activated complexes and lowers down the activation energy, but the difference in energy of reactant and product remains the same."
- **Explanation**: A catalyst speeds up a reaction by lowering the activation energy, which is the energy barrier that must be overcome for a reaction to occur. However, it does not change the inherent energy levels of the reactants and products.

### Step 3: Analyze the Relationship
Now, we need to determine if the reason correctly explains the assertion.
- **Analysis**: Since the catalyst lowers the activation energy but does not affect the enthalpy change (the difference in energy between reactants and products), the reason supports the assertion.

### Step 4: Conclusion
Both the assertion and the reason are correct, and the reason provides a correct explanation for the assertion.
- **Final Answer**: Therefore, the correct option is that both assertion and reason are correct, and the reason is the correct explanation of the assertion.

### Summary of Steps:
1. Identify and understand the assertion.
2. Identify and understand the reason.
3. Analyze if the reason supports the assertion.
4. Conclude the relationship between the assertion and reason.