Among the 3d-series transition metals, the element that exhibits the largest number of oxidation states is:

Manganese has the configuration $[\mathrm{Ar}]3d^5 4s^2$. All five 3d electrons and the two 4s electrons can participate in bonding, so it can lose anywhere from one to seven electrons, giving oxidation states $+2$ through $+7$. Examples: $\mathrm{Mn}^{2+}$ in $\mathrm{MnSO_4}$, $\mathrm{Mn}^{4+}$ in $\mathrm{MnO_2}$, $\mathrm{Mn}^{7+}$ in $\mathrm{KMnO_4}$.