Assertion: If bob of a simple pendulum is kept in a horizontal electric field, its period of oscillation will remain same.
Reason: If bob is charged and kept in horizontal electric field, then the time period will be decreased.

Options

A. If both assertion and Reason are correct but Reason is the correct explanation of Assertion.
B. If both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.
C. If Assertion is correct but Reason is incorrect.
D. If assertion is incorrect and reason is correct.

Explanation

To solve the question, we will analyze the assertion and the reason given in the problem step by step.

### Step 1: Understanding the Assertion
The assertion states that if the bob of a simple pendulum is kept in a horizontal electric field, its period of oscillation will remain the same.

**Explanation:**
- The period of a simple pendulum is given by the formula:
\[ T = 2\pi \sqrt{\frac{L}{g}} \]
where \(L\) is the length of the pendulum and \(g\) is the acceleration due to gravity.
- In a horizontal electric field, if the bob is neutral (not charged), the only forces acting on it are its weight (mg) and the tension in the string. The electric field does not affect the gravitational force acting on the bob.
- Therefore, the effective gravitational force remains unchanged, and thus the period of oscillation remains the same.

### Step 2: Understanding the Reason
The reason states that if the bob is charged and kept in a horizontal electric field, then the time period will be decreased.

**Explanation:**
- When the bob is charged and placed in a horizontal electric field, it experiences an additional force due to the electric field, given by \(F = QE\), where \(Q\) is the charge and \(E\) is the electric field strength.
- The net force acting on the bob will now be a combination of the gravitational force and the electric force. The effective gravitational force can be modified to account for the electric force:
\[ g_{\text{effective}} = \sqrt{g^2 + \left(\frac{QE}{m}\right)^2} \]
- The period of oscillation now becomes:
\[ T = 2\pi \sqrt{\frac{L}{g_{\text{effective}}}} \]
- As the electric field strength \(E\) increases, the effective gravitational force \(g_{\text{effective}}\) increases, leading to a decrease in the period \(T\).

### Step 3: Conclusion
- Both the assertion and the reason are true.
- However, the reason does not correctly explain the assertion because the assertion is true for a neutral bob, while the reason applies only when the bob is charged.

### Final Answer
- Assertion: True
- Reason: True
- Conclusion: The assertion and reason are both true, but the reason is not the correct explanation of the assertion.