Answer:
The mass do not change.
Step-by-step explanation:
The air that is trapped inside the balloon starts expanding when the balloon is heated. This happen because the air molecules move away from each other. This results in the expansion of the balloon.
In the whole process, the mass of air trapped inside balloon do not change. Also, there is no change in the balloon itself. Thus mass remains the same. Though, the volume changes due to expansion of the balloon.