The tendency of formation of basic oxide increases when shifting down in a group.
A basic oxide is formed when an element combines with oxygen and produces a basic solution when dissolved in water.
The trend of basic oxide formation in a group is dependent on the size of the atom and its valence electrons.
As we move down a group, the atomic size increases, and the valence electrons are located farther from the nucleus.
The increase in atomic size and distance of valence electrons from the nucleus leads to a decrease in the effective nuclear charge experienced by the valence electrons.
As a result, the valence electrons are less strongly attracted to the nucleus, making it easier for them to form a basic oxide.