i think basic strength can be judged on the basis of ease of giving OH^- with water on reacn.

As Ba is lowest in 2nd group. ther4 bond between Ba & O is most ionic amongst given. As we go above basic strenth dec. as covalent char. incr. in bond.(ther4 less interaction with water)

since water is apolar solvent, it readily reacts with ionic compounds(giving OH^-) hence, b is ans