Why do transition elements show variable oxidation states?

Why do transition elements show variable oxidation states?
(i) Name the element showing maximum number of oxidation states among the first series of transition metals from Sc (Z = 21) to Zn (Z = 30).
(ii) Name the element which shows only +3
oxidation state.

Transition elements show variable oxidation states because they have electrons in d-orbitals (d-orbital is the outermost orbital of transition element).
(i) Mn (manganese) shows the maximum number of oxidation states.
(ii) Scandium shows only +3 oxidation state.