Final answer:
The heat absorbed during the isothermal expansion of an ideal gas is negative.
Step-by-step explanation:
The correct option is d. Negative; internal energy will increase.
During an isothermal expansion of an ideal gas, the temperature remains constant. As a result, the internal energy of the gas does not change since it depends only on the temperature. However, work is done on the gas during expansion, which means energy is transferred from the surrounding to the gas. Therefore, heat is absorbed during the isothermal expansion.