Final answer:
When heat energy is added to a system under isothermal conditions, it leads to an increase in internal energy.
Explanation:
Heat energy added to a system under isothermal conditions appears as an increase in internal energy. This means that the heat added causes a change in the internal energy of the system, without a change in temperature.
In this scenario, the option C, Increase in internal energy, is the correct choice. When heat is added to a system under isothermal conditions, the energy manifests as an increase in the internal energy of the system, affecting its overall behavior.
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