Assertion : When two positive point charges move away from each other, then their electrostatic potential energy decreases.
Reason : Change in potential energy between two points is equal to the work done by electrostatic forces.

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 correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If Assertion is false but Reason is true.

Answer

To solve the question, we need to analyze both the assertion and the reason given.

### Step 1: Understanding the Assertion
The assertion states that when two positive point charges move away from each other, their electrostatic potential energy decreases.

- The electrostatic potential energy (U) between two point charges \( q_1 \) and \( q_2 \) separated by a distance \( r \) is given by the formula:
\[ U = \frac{k \cdot q_1 \cdot q_2}{r} \]
where \( k \) is Coulomb's constant.

- Since both charges are positive (\( q_1 > 0 \) and \( q_2 > 0 \)), the potential energy is positive.

- As the distance \( r \) increases (the charges move away from each other), the value of \( U \) decreases because \( U \) is inversely proportional to \( r \). Thus, the assertion is **true**.

### Step 2: Understanding the Reason
The reason states that the change in potential energy between two points is equal to the work done by electrostatic forces.

- The work done by electrostatic forces when moving a charge in an electric field is related to the change in potential energy. For conservative forces, the work done (W) is given by:
\[ W = -\Delta U \]
where \( \Delta U \) is the change in potential energy.

- This means that the work done by the electrostatic force is equal to the negative of the change in potential energy. Therefore, the reason is also **true**.

### Step 3: Evaluating the Relationship
While both the assertion and the reason are true, we need to determine if the reason correctly explains the assertion.

- The assertion discusses the behavior of potential energy as charges move apart, while the reason discusses the relationship between work done and change in potential energy but does not specifically explain why the potential energy decreases when the charges move apart.

### Conclusion
- The assertion is true: the potential energy decreases as the charges move apart.
- The reason is true: the change in potential energy is related to the work done by electrostatic forces.
- However, the reason does not provide a correct explanation for the assertion.

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