Compare the stability of +2 oxidation state for the elements of the first transition series.

in the first row of transition element the oxidation state increases from +2 to +7 and the large variabilty is shown by Mn(+2 to +7).from Mn to Zn the number of oxidation state decreases due to decrease in the number of avilable unpaired electron
the relative stability of +2 oxidation state decreases from top to bottom because it becomes difficult to remove the third electron from the d orbital