What characteristic do transition metals often hold regarding their ionic states?

Transition metals are known for their ability to exhibit multiple oxidation states due to the involvement of their d-electrons in bonding. This characteristic arises from the relatively close energy levels of the d-orbitals and their ability to lose different numbers of electrons when forming ions. For example, iron can exist in oxidation states of +2 and +3, while copper can exist in +1 and +2 states. This variability in oxidation states allows transition metals to engage in a wide range of chemical reactions and form complex ions, which is a defining feature of their chemistry.

The other options do not accurately describe the behavior of transition metals. They are not always diatomic; many transition metals can be found as monatomic or in various forms. Additionally, transition metals frequently form ions rather than rarely doing so, and while they primarily form cations, they can also participate in complex formation that involves anions. Thus, the correct choice reflects the unique complexity and versatility of transition metal chemistry.