A : Ratio of empirical formula mass and molecular formula mass must be a natural number.
R: `"Molecular formula mass" = nX, X is "empirical formula mass", where n is the simplest whole number.

Options

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

Answer

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

**Assertion (A):** The ratio of empirical formula mass and molecular formula mass must be a natural number.

**Reason (R):** "Molecular formula mass" = nX, where X is "empirical formula mass" and n is the simplest whole number.

### Step-by-Step Solution:

1. **Understanding Empirical and Molecular Formula:**
- The empirical formula represents the simplest whole number ratio of the elements in a compound.
- The molecular formula represents the actual number of atoms of each element in a molecule of the compound.

2. **Defining the Masses:**
- Let the empirical formula mass be denoted as \( X \).
- The molecular formula mass can be expressed as \( n \times X \), where \( n \) is a whole number that indicates how many times the empirical formula fits into the molecular formula.

3. **Finding the Ratio:**
- The ratio of molecular formula mass to empirical formula mass can be expressed as:
\[ \text{Ratio} = \frac{\text{Molecular Formula Mass}}{\text{Empirical Formula Mass}} = \frac{nX}{X} = n \]
- Here, \( n \) is a whole number (it can be a natural number as well).

4. **Conclusion on the Assertion:**
- Since \( n \) is a whole number, the ratio of empirical formula mass to molecular formula mass is indeed a natural number (provided \( n \) is greater than or equal to 1).
- Thus, the assertion (A) is true.

5. **Analyzing the Reason:**
- The reason states that the molecular formula mass is equal to \( n \times X \), which is correct.
- However, the reason uses the term "simplest whole number" which may not directly explain why the ratio must be a natural number.

6. **Final Evaluation:**
- Both the assertion and the reason are true statements.
- However, the reason does not correctly explain the assertion since it introduces the concept of "simplest whole number" instead of directly addressing the natural number aspect.

### Conclusion:
- The correct answer is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion.