The answer is C. Air inside a balloon. This is because gases are much more affected by thermal expansion and contraction compared to liquids and solids. The reason behind this is that gas molecules are very loosely packed, so even the slightest changes in temperature have a big effect on its pressure and volume. If the air inside a balloon has a temperature rise, the tendency is for the balloon to expand and potentially explode.
Liquids are also affected by thermal expansion or contraction, but not as much as gases. Solids tend to maintain their shape under thermal expansion or contraction.
