A : When we change the unit of a measurement of a quantity, its numerical value changes.
R : The product of numerical value of the physical quantity and unit for a quantity remain constant.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statements, then mark (4).

Answer

To solve the question, we need to analyze the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that when we change the unit of measurement of a quantity, its numerical value changes.
- This is true because different units represent the same physical quantity in different scales. For example, 1 meter is equivalent to 100 centimeters. Therefore, when we convert from meters to centimeters, the numerical value changes from 1 to 100.

2. **Understanding the Reason (R)**:
- The reason states that the product of the numerical value of the physical quantity and the unit for a quantity remains constant.
- This is also true. The relationship can be expressed as:
\[ \text{Magnitude} \times \text{Unit} = \text{Constant} \]
- For instance, if we have a length of 1 meter, we can express it as:
\[ 1 \text{ m} = 100 \text{ cm} \]
Here, the numerical value changes, but the physical quantity (length) remains the same.

3. **Conclusion**:
- Both the assertion and the reason are correct. The assertion correctly states that changing the unit changes the numerical value, and the reason correctly explains that the product of the numerical value and the unit remains constant for the same physical quantity.
- Therefore, the correct answer is that both the assertion and the reason are true and correctly explained.

### Final Answer:
Both assertion (A) and reason (R) are correct.