In the formula `p = 2/3 E`, the term (E) represents translational kinetic energy per unit volume of gas.
In case of monoatomic gas, translational kinetic energy and total kinetic energy are equal.

Options

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

Explanation

To solve the problem, we need to analyze the relationship between pressure (P) and the translational kinetic energy (E) of a monoatomic gas, as well as understand the implications of the degrees of freedom for such gases.

### Step-by-Step Solution:

1. **Understanding the Formula**:
The formula given is \( P = \frac{2}{3} E \), where \( E \) represents the translational kinetic energy per unit volume of the gas.

2. **Kinetic Energy in Gases**:
The total kinetic energy of a gas can be divided into translational, rotational, and vibrational components. For a monoatomic gas, the only relevant component is the translational kinetic energy because it has no rotational or vibrational degrees of freedom.

3. **Degrees of Freedom**:
In the case of a monoatomic gas, the degrees of freedom are 3, corresponding to the three translational motions (along the x, y, and z axes). Thus, all kinetic energy is translational.

4. **Translational vs. Total Kinetic Energy**:
Since a monoatomic gas has no rotational or vibrational energy, the total kinetic energy is equal to the translational kinetic energy. Therefore, we can say:
\[ \text{Total Kinetic Energy} = \text{Translational Kinetic Energy} \]
This confirms that for monoatomic gases, \( E \) is indeed equal to the translational kinetic energy.

5. **Conclusion**:
Both the assertion (that \( E \) represents translational kinetic energy per unit volume) and the reason (that in case of monoatomic gas, translational kinetic energy and total kinetic energy are equal) are true. However, the reason does not explain the assertion directly. Thus, the correct conclusion is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
Both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

---