Molecule size is the variable. The ability of the material to diffuse is the variable.

Molecule size - independent variable

The ability of the material to diffuse - dependent variable

Explanation:

The lab experiment was supposed to see the effect of molecule size on the ability of the material to diffuse.

The independent variable would be the molecule size because to determine this, one would need to try different sizes of the molecule to see if the size would have an effect on the diffusion.

The ability of the material to diffuse is the dependent variable because the results would depend on the independent variable or the size of the molecule.