Use the first law of thermodynamics

The first law is $\Delta Q = \Delta U + \Delta W$ where:

• $\Delta Q$ = heat supplied,
• $\Delta U$ = change in internal energy,
• $\Delta W$ = work done by the system.

Relation between internal energy and temperature

For an ideal gas, internal energy depends only on temperature. Hence, $\Delta U \propto \Delta T$ So if internal energy remains constant, then $\Delta U = 0 \implies \Delta T = 0$

Identify the thermodynamic process with constant temperature

A process in which temperature remains constant is called an isothermal process.

Check the options

• A: Isobaric $\rightarrow$ pressure constant, not necessarily $\Delta U=0$
• B: Isochoric $\rightarrow$ volume constant, not necessarily $\Delta U=0$
• C: Adiabatic $\rightarrow$ no heat exchange, but temperature and internal energy can change
• D: Isothermal $\rightarrow$ temperature constant, so for an ideal gas $\Delta U=0$

Conclusion

Therefore, the thermodynamic process in which internal energy remains constant is: $\boxed{\text{Isothermal}}$

Comparison with stored answer

Stored correct answer: D

Derived answer: D

These match.