Here's why:

* Ionic Character: CaO has a very high ionic character due to the large difference in electronegativity between calcium (a metal) and oxygen (a nonmetal). This means that the Ca-O bond is strongly polarized, with the oxygen atom carrying a significant negative charge.

* Lattice Energy: CaO has a very high lattice energy, which is the energy required to separate one mole of a solid ionic compound into its gaseous ions. The high lattice energy indicates strong electrostatic attractions between the ions.

* Solubility: CaO is relatively soluble in water, forming calcium hydroxide (Ca(OH)2), a strong base.

In summary: The combination of strong ionic character, high lattice energy, and significant solubility in water makes CaO the strongest basic oxide.