Answer
C. Decrease in size
Step-by-step explanation
The volume of the balloon filled up with helium molecules will increase as the temperature increase. This is because as the temperature changes, the helium molecules are constantly moving and colliding due to kinetic energy in them.
But as soon as the balloon is taken outside on a cold day, the helium gas molecules inside it loses energy to achieve equal temperature as the surroundings (thermal equilibrium) and they start getting collected together as they aren’t moving anymore (no kinetic energy) so the balloon shrinks, thereby the volume of the ballon decrease in size.