Which solid is likely to be least soluble in water? and why?

SiO2, because this substance is non-polar and does not dissociate into ions. C6H12O6 does not form any ions, but the molecule is very polar, which makes it soluable. I2 is also non-polar, but can form free radicals. MgBr2 and CaCO3 are salts and dissociate into water soluable ions.