how does activity explain the differences in the observed solubility of Ca(IO3)2 into each of these solvents

Your question is not clear but if you can explain yourself i would be glad to .  just trying to : ))

Remember that rust prevented by placing a sacrificial electrode in electrical contact with the iron. the sacrificial electrode must be composed of a metal that  oxidizes  more easily than iron (that is, it must be below iron in table 18.1).