This question begins with something, you should know: molar mass from water is aproximately 18 g/m, so if 18 grams of water are contained in 1 mole, the 40 grams occuped 2.22 moles. As you see, opcion a is the best!
The radius of a chlorine ion is larger than the radius of a chlorine atom because the effective nuclear charge decreases, therefore the inward force decreases, increasing the ionic radius.