A : Speed of sound in moist air is more than its speed in dry air .
R : Dry air is denser than moist air at atmospheric pressure .

Answer

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

**Assertion (A):** Speed of sound in moist air is more than its speed in dry air.
**Reason (R):** Dry air is denser than moist air at atmospheric pressure.

### Step-by-Step Solution:

1. **Understand the relationship between speed of sound and density:**
The speed of sound in a gas can be expressed with the formula:
\[ v = \sqrt{\frac{\gamma P}{\rho}} \]
where \( v \) is the speed of sound, \( \gamma \) is the adiabatic index (ratio of specific heats), \( P \) is the pressure, and \( \rho \) is the density of the gas.

2. **Analyze the effect of moisture on air density:**
When air becomes moist, water vapor replaces some of the nitrogen and oxygen molecules in the air. The molecular weight of water vapor is less than that of dry air. Specifically, the density of water vapor is approximately \( \frac{5}{8} \) times that of dry air. Therefore, as humidity increases, the overall density of the air decreases.

3. **Relate density to speed of sound:**
Since the speed of sound is inversely proportional to the square root of the density (\( v \propto \frac{1}{\sqrt{\rho}} \)), a decrease in density (due to increased humidity) will result in an increase in the speed of sound. Thus, the speed of sound in moist air is greater than in dry air.

4. **Evaluate the assertion (A):**
Based on the analysis above, we can conclude that the assertion is true: the speed of sound in moist air is indeed greater than in dry air.

5. **Evaluate the reason (R):**
The reason states that dry air is denser than moist air at atmospheric pressure, which is also true. As established, the presence of water vapor reduces the overall density of the air.

6. **Determine the relationship between A and R:**
Since the reason (R) correctly explains why the assertion (A) is true, we can conclude that both statements are true, and R is the correct explanation for A.

### Final Conclusion:
Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the correct option is that both A and R are true, and R is the correct explanation of A.

---