Which of the following describes an isothermal process?

An isothermal process is defined as one that occurs at constant temperature. In such a process, the internal energy of an ideal gas remains unchanged since the temperature, which is a measure of the average kinetic energy of the gas particles, does not vary.

To maintain a constant temperature, heat must be added or removed from the system in response to changes in pressure or volume. This is a key characteristic of isothermal processes in thermodynamics, particularly when dealing with gases in an ideal scenario.

In contrast, processes that occur at increasing temperature, at constant pressure, or at constant volume do not meet the criteria for being isothermal. An increase in temperature indicates a change in the internal energy of the system, while constant pressure and constant volume describe different thermodynamic processes (isobaric and isochoric, respectively) that do not inherently require temperature to remain constant. Thus, recognizing that an isothermal process means constant temperature is crucial in thermodynamic studies.