Select the correct answer from each drop-down menu. Why do some transition metals have multiple oxidation states?

Transition metals have multiple oxidation states because of their (a. completely filled b. partially filled c. empty) (s, p, d, or f) sublevel.

because of energy difference between (n1)d and ns orbitals (sub levels) and involvement of both orbital in bond formation

Explaination:

the reason is that there is a difference in energy of orbitals of an atom of transition metal, so there (n1)d orbitals and there ns orbitals both make a bond and for this purpose they lose an electron that is why both sublevels shows different oxidation state.  

Hope this helped you

Transition metals have multiple oxidation states because of their  

[ partially filled ] [ d ] sublevel.