Which transition element does not exhibit variable oxidation state?

Because scandium (Z=21) possesses partially full d-orbitals in the ground state 

($3d^{1}4s^{\mathit{2}}$), while not having changing oxidation states, it is classified as a transition element.

Every element in the d-blocks has a different valency or oxidation number. However, because scandium's ground state d-orbitals are only half filled, scandium does not appear. The 3d and 4s orbitals of scandium have 1 and 2 electrons, respectively. A particularly stable inert gas configuration is created by taking three electrons out of the atom's d and s orbitals. To create a ${\mathrm{Sc}}^{3+}$ ion, it loses one electron from the d orbital and two from the s orbital.