( A ) $Co^{3+} < Fe^{3+} < Cr^{3+} < Sc^{3+}$ : stability in aqueous solution
( B ) $Ni^{2+} < Co^{2+} < Fe^{2+} < Mn^{2+}$ : ionic size
( C ) $Sc < Ti < Cr < Mn$ : number of oxidation states
( D ) $Cr^{2+} < V^{2+} < Fe^{2+} < Mn^{2+}$ : paramagnetic behaviour
