Answer:
Acetone
Step-by-step explanation:
Chromatography has to do with the separation of substances by eluting the components of the solute using a mobile phase. The composition of the mixture to be separated decides the mixture of solvents (mobile phase) that are to be used for elution.
In this case the mixture to be separated comprises of substance A which is polar and substance B which is non polar. It is observed that the mobile phase moves B up the plate but not the polar substance, A. This means that a polar solvent such as acetone should be added to the mobile phase in order to achieve better elution of A.