$Sc^{3+}$ has stable electronic configuration (vacant d-orbital), therefore $Sc^{3+}$ is more stable than $Sc^{+}$. $Fe^{3+}$ is more stable than $Fe^{2+}$ due to half filled d-orbitals.$Mn^{2+}$ is more stable than $Mn^{3+}$ due to half filled d-orbitals.$V^{5+}$ is more stable (due to vacant d-orbital) than $V^{3+}$.