Just looking at the table it appears that the ions with notable delta G values have larger radii. Based on just that, I would choose b) the size of the ion.
Yeah, if Na+ was able to bind the channel then the answer would be the charge of the ion, but the fact that Na+ can't bind along with Ca2+ and Mg2+ eliminates the charge option. The only other viable option is size, since the permeable ions all are larger