Suppose a balloons was laying by the window at night. The next day, when the sun came up, it warmed the gas (air) that was in the balloon. What would be true about the density of the air in the balloon?

Negligible change.

Explanation:

The gas in the balloon after getting exposed to the sunlight experiences a rise in heat of both the balloon and the air. Independently, air on being heated it expands and the volume increases and hence its density would reduce. Contrarily, when the rubber balloon is heated independently it would shrink on heating because its bods get stronger hence the chain of molecules shrink. Therefore we will have a combined effect of the both the matters due to increased heat and hence will nearly nullify each other and hence there will be negligible change in the density of the gas in the balloon because the volume of balloon tries to shrink on being heated.