Variable oxidation numbers are possible for:

Variable oxidation numbers are a characteristic feature of transition metals. Iron (Fe) is a transition metal that commonly exhibits multiple oxidation states, most notably +2 (ferrous) and +3 (ferric). This variability arises because the energy levels of the 4s and 3d orbitals are very close, allowing electrons from both shells to participate in chemical bonding. In contrast, Group 1 elements like Sodium (Na) and Lithium (Li) consistently exhibit a +1 oxidation state in their compounds. Similarly, Group 2 elements like Calcium (Ca) typically show a fixed +2 oxidation state. While uncombined elements have an oxidation state of zero, only iron among the given options frequently forms stable compounds in different positive oxidation states.

Sources

1. [1] http://www.macw.ac.in/downloads/files/n5e880c6ae27ce.pdf