Explain briefly how `+2` state become more and stable in the first half of the first row transition elements with increasing atomic number?

Explanation

In all the element listed, with the removal of valenece 4s electrons (+2 oxidation state ), the 3d-orbitals get gradually occupied. Since the number of empty d-orbitals decrease or the number of unpaired electrons in 3d orbitals increase, the stability of the cations `(M^(2+))` increase from `Sc^(2+)` to `Mn^(2+)`