What is a characteristic feature of transition metals?

Transition metals are known for their ability to exhibit multiple oxidation states, which is a distinctive characteristic of this group of elements. This property arises due to the involvement of both the outermost s and the penultimate d orbitals in chemical bonding. As a result, transition metals can lose varying numbers of electrons from these d orbitals, leading to the possibility of forming ions with different charges. This versatility in oxidation states allows transition metals to participate in a variety of chemical reactions and form complex compounds with different properties.

For example, iron can exist in +2 and +3 oxidation states, copper in +1 and +2, and chromium in +3 and +6 states, among others. This characteristic is crucial for their role as catalysts and in the formation of coordination complexes.

The other options do not accurately reflect the properties of transition metals, as they are known for having variable oxidation states rather than fixed ones, they frequently form colored compounds due to d-d electron transitions, and they can exist in various physical states, not solely in solid form.