In an isothermal process, ............. Remains constant.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Isothermal Process**: An isothermal process is a thermodynamic process in which the temperature of the system remains constant throughout the process. This means that any heat added to the system is used to do work, rather than changing the internal energy of the system.

2. **Key Characteristics**: In an isothermal process, the internal energy of an ideal gas depends only on its temperature. Since the temperature is constant, the internal energy also remains constant.

3. **Examples of Isothermal Processes**: Common examples include the melting of ice at 0°C and the operation of a refrigerator, where the temperature is maintained constant during the heat exchange process.

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