Assertion A diabatic bulk modulus of an ideal gas in more than its isothermal bulk modulus.
Reason Both the modulii aer proportional to the pressure of gas at that moment.

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.

Explanation

To solve the question, we need to analyze the assertion and the reason provided, and then determine their validity.

### Step-by-Step Solution:

1. **Understanding Bulk Modulus**:
- The bulk modulus (B) of a substance measures its resistance to uniform compression. It is defined as:
\[ B = -\frac{P}{\Delta V/V_0} \]
- Here, \( P \) is the pressure, \( \Delta V \) is the change in volume, and \( V_0 \) is the original volume.

2. **Isothermal Process**:
- For an isothermal process (constant temperature), the relationship between pressure and volume is given by Boyle's Law:
\[ PV = \text{constant} \]
- From this, we can derive that:
\[ \frac{\Delta P}{P} + \frac{\Delta V}{V} = 0 \]
- Rearranging gives:
\[ \frac{\Delta P}{\Delta V} = -\frac{P}{V} \]
- Substituting this into the bulk modulus formula for isothermal conditions gives:
\[ B_{iso} = -V_0 \left(-\frac{P}{V}\right) = P \]

3. **Adiabatic Process**:
- For an adiabatic process (no heat exchange), the relationship is given by:
\[ PV^\gamma = \text{constant} \]
- This leads to:
\[ \frac{\Delta P}{P} + \gamma \frac{\Delta V}{V} = 0 \]
- Rearranging gives:
\[ \frac{\Delta P}{\Delta V} = -\gamma \frac{P}{V} \]
- Substituting this into the bulk modulus formula for adiabatic conditions gives:
\[ B_{adi} = -V_0 \left(-\gamma \frac{P}{V}\right) = \gamma P \]

4. **Comparison of Bulk Moduli**:
- From the calculations:
\[ B_{iso} = P \quad \text{and} \quad B_{adi} = \gamma P \]
- Since \( \gamma > 1 \) (for any ideal gas), it follows that:
\[ B_{adi} > B_{iso} \]
- Therefore, the assertion that the adiabatic bulk modulus of an ideal gas is greater than its isothermal bulk modulus is **true**.

5. **Analyzing the Reason**:
- The reason states that both moduli are proportional to the pressure of the gas at that moment.
- From our equations:
- \( B_{iso} = P \) (proportional to pressure)
- \( B_{adi} = \gamma P \) (also proportional to pressure)
- Thus, the reason is also **true**.

6. **Conclusion**:
- Both the assertion and the reason are true, but the reason does not correctly explain the assertion. The assertion is about the relationship between the two bulk moduli, while the reason only states their proportionality to pressure.

### Final Answer:
- The assertion is true, and the reason is true but does not provide a correct explanation for the assertion. Therefore, the correct option is **B**.