Q.Which of the following has highest hydration energy

Group 2 -- beryllium, magnesium, calcium, strontium, barium and radium -- are the alkaline earth metals (all except Be, whose oxides/hydroxides are alkaline and occur, like the rest, in the earth's crust). Their common valence-shell configuration is [Noble gas]ns².

Calcium is the earth's fifth most abundant element and magnesium the eighth; beryllium is rare and radium the rarest (a decay product of uranium). Because of their higher nuclear charge, alkaline earth metal atomic/ionic radii are smaller than the corresponding alkali metals of the same period, though (as with alkali metals) radii still increase down the group. Every group 2 element shows a single oxidation state, +2, by losing both valence electrons. …