Assertion : The size and shape of the rigid body remains unaffected under the effect of external force.
Reason : The distance between two particles remains constant in a rigid body.

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.

Answer

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

### Step 1: Understand the Assertion
The assertion states: "The size and shape of the rigid body remains unaffected under the effect of external force."

- A rigid body is defined as an object that does not deform; that is, its shape and size remain constant regardless of the forces applied to it. This means that when external forces act on a rigid body, it does not change its dimensions or shape.

### Step 2: Understand the Reason
The reason states: "The distance between two particles remains constant in a rigid body."

- This statement is true because, by definition, a rigid body is one in which the distance between any two particles (or points) within the body does not change, even when external forces are applied. This characteristic is what allows the rigid body to maintain its size and shape.

### Step 3: Analyze the Relationship
Now, we need to determine if the reason correctly explains the assertion.

- Since the reason (the constant distance between particles) is a fundamental property of rigid bodies, it directly supports the assertion that the size and shape of a rigid body remain unaffected by external forces. The reason provides a clear explanation of why the assertion is true.

### Conclusion
Both the assertion and the reason are true, and the reason is indeed the correct explanation for the assertion.

### Final Answer
Both the assertion and the reason are true, and the reason is the correct explanation for the assertion.