What is the single factor that classifies an element as a transition metal? a. only the "s" orbitals are filled b. the highest occupied orbital is aa "d" orbital c. all orbitals are filled with electrons except the "s" orbitals d. aall "s" and "p" orbitals are filled

The answer is b) the highest occupied orbital is a “d”orbital.
Transition metals are metals where the highest energy electrons partially fill the d subshells. There are some elements with complete d subshells but on forming cations they have incomplete d subshells.
These transition metals have some properties that are different from the other metals .