Assertion : `E_(a)` of the forward reaction is higher than that of backward reaction in a reversible endothermic reaction.
Reason : Increasing the tempreature of the substance increases the fraction of molecules which collide with energies greater than `E_(a)`.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the assertion and reason question regarding activation energy in a reversible endothermic reaction, we can break it down into a step-by-step analysis.

### Step 1: Understand the Assertion
The assertion states that the activation energy (Ea) of the forward reaction is higher than that of the backward reaction in a reversible endothermic reaction.

- **Explanation**: In an endothermic reaction, energy is absorbed as the reaction proceeds from reactants to products. The products have a higher energy level than the reactants. Therefore, the activation energy for the forward reaction (reactants to products) must be greater than that for the backward reaction (products to reactants).

### Step 2: Understand the Reason
The reason states that increasing the temperature of the substance increases the fraction of molecules that collide with energies greater than Ea.

- **Explanation**: This statement is true. According to the Arrhenius equation, as temperature increases, the kinetic energy of molecules increases, leading to a higher fraction of molecules having sufficient energy to overcome the activation energy barrier.

### Step 3: Analyze the Relationship between Assertion and Reason
Now we need to determine if the reason correctly explains the assertion.

- **Assertion**: True (Ea for forward > Ea for backward in an endothermic reaction).
- **Reason**: True (Increasing temperature increases the fraction of molecules with energy > Ea).

However, the reason does not explain why the activation energy of the forward reaction is higher than that of the backward reaction. The correct explanation involves the concept of threshold energy (ETH) of reactants and products, where the threshold energy of reactants is less than that of products in an endothermic reaction.

### Conclusion
Both the assertion and reason are true, but the reason does not provide the correct explanation for the assertion. Therefore, the correct option is:

**Option B**: Both assertion and reason are true, but reason is not the correct explanation of the assertion.