Internal energy change during a reversible is isothermal expansion of an ideal is :-

Options

A. Always negative
B. Always positive
C. Zero
D. May be positive or negative

Explanation

To solve the question regarding the internal energy change during a reversible isothermal expansion of an ideal gas, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Internal Energy**:
- Internal energy (U) of an ideal gas is a function of temperature. This means that any change in internal energy is directly related to a change in temperature.

2. **Identify the Type of Process**:
- The process described is an isothermal expansion. "Isothermal" means that the temperature remains constant throughout the process.

3. **Relate Temperature to Internal Energy**:
- Since internal energy is a function of temperature, if the temperature does not change (as in an isothermal process), the internal energy must also remain constant.

4. **Determine the Change in Internal Energy**:
- If the internal energy is constant, the change in internal energy (ΔU) is zero. Mathematically, this can be expressed as:
\[ \Delta U = U_{\text{final}} - U_{\text{initial}} = 0 \]

5. **Conclusion**:
- Therefore, during a reversible isothermal expansion of an ideal gas, the change in internal energy is zero.

### Final Answer:
The internal energy change during a reversible isothermal expansion of an ideal gas is **zero**.