A: For a given solution (density `1 gm//ml`), molality is greater than molarity.
R: Molarity involves volume of solution while molality involves mass of solvent.

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

### Step-by-Step Solution:

1. **Understanding the Definitions**:
- **Molarity (M)** is defined as the number of moles of solute per liter of solution.
\[ \text{Molarity (M)} = \frac{\text{Number of moles of solute}}{\text{Volume of solution in liters}} \]
- **Molality (m)** is defined as the number of moles of solute per kilogram of solvent.
\[ \text{Molality (m)} = \frac{\text{Number of moles of solute}}{\text{Mass of solvent in kg}} \]

2. **Given Information**:
- The density of the solution is given as \(1 \text{ gm/ml}\), which means \(1 \text{ liter}\) of solution has a mass of \(1000 \text{ grams}\) or \(1 \text{ kg}\).

3. **Comparing Molarity and Molality**:
- Since the density is \(1 \text{ gm/ml}\), \(1 \text{ liter}\) of the solution weighs \(1 \text{ kg}\).
- If we assume \(x\) moles of solute are present in this \(1 \text{ liter}\) of solution, the molarity can be expressed as:
\[ M = \frac{x \text{ moles}}{1 \text{ liter}} = x \]
- For molality, we need to consider the mass of the solvent. If the solution is \(1 \text{ kg}\) and we assume that the solute contributes a negligible amount to the mass, the mass of the solvent is approximately \(1 \text{ kg}\) (assuming a small amount of solute).
- Thus, the molality can be expressed as:
\[ m = \frac{x \text{ moles}}{1 \text{ kg}} = x \]

4. **Conclusion**:
- In this case, when the density is \(1 \text{ gm/ml}\), the mass of the solution is equal to the volume, which means that the mass of the solvent is less than the total mass of the solution. Therefore, since molality is based on the mass of the solvent, it will be greater than molarity.
- Thus, the assertion (A) is true: "For a given solution (density \(1 \text{ gm/ml}\)), molality is greater than molarity."
- The reason (R) is also true: "Molarity involves volume of solution while molality involves mass of solvent."

### Final Answer:
Both the assertion and the reason are correct, and the reason correctly explains the assertion. Therefore, the correct option is **Option A**.