All the given transition metals are in the +2 oxidation state (monoxides). For transition metal oxides in the same oxidation state, the basic character generally decreases from left to right across a period. This is because the atomic size decreases and the effective nuclear charge increases, leading to an increase in the ionization enthalpy and a decrease in the metallic character. Consequently, the covalent character of the metal-oxygen bond increases, which reduces the basic nature of the oxide. Since the atomic numbers are in the order Ti (22) < V (23) < Cr (24) < Fe (26), the basic character decreases in the same order: TiO > VO > CrO > FeO.