Which of the four balloons will shrink?
The relative molecular mass ($M_r$) of the gas determines the rate of diffusion of the gases from a region of higher concentration to a region of lower concentration.  

The smaller the $M_r$ of the gas, the greater the rate of diffusion (movement of gas molecules).


  $M_r$ of $\ce{CO2}$ = 40  


$M_r$ of $\ce{H2}$ = 2 (Balloon A)
Since hydrogen gas has a smaller $M_r$ than carbon dioxide gas, more hydrogen gas diffuse out of the balloon than carbon dioxide gas into the balloon per unit time. Hence, this balloon will shrink.

$M_r$ of $\ce{N2O}$ = 44  (Balloon B)
Since nitrous oxide gas has a greater $M_r$ than carbon dioxide gas, more carbon dioxide gas diffuse into the balloon than nitrous oxide gas out of the balloon per unit time. Hence, this balloon will inflate  

$M_r$ of $\ce{CO}$ = 28  (Balloon C)
This balloon will shrink (same logic as $\ce{H2}$).  

$M_r$ of $\ce{CO2}$ = 40  (Balloon D)
Since the balloon and the tank both contains $\ce{CO2}$ (same $M_r$ of gases), there is no net diffusion of gases in/out of the balloon, thus the balloon will not inflate/shrink (theoretically).