Final answer:
When thermal energy is added to a partially deflated balloon, the average kinetic energy of the gas particles increases but the mass of the balloon remains the same unless gas is added or escapes.
Step-by-step explanation:
When thermal energy is added to a partially deflated balloon, the average kinetic energy of the gas particles inside increases, making the balloon hotter. This does not change the mass of the gas inside the balloon unless the gas escapes or additional gas is introduced. The correct answer to the question is C) The mass of the balloon remains the same.
The addition of thermal energy would likely cause an expansion of the balloon as the increased particle motion results in increased pressure, and if the balloon is flexible, also leads to increased volume. The mass of the balloon, however, is a measure of the amount of matter it contains and does not change simply due to temperature changes unless there is a transfer of matter.