Concept:Variable oxidation numbers occur when an element can lose different numbers of electrons, typically due to availability of d-orbitals.
Explanation:Iron is a transition metal with electronic configuration [Ar] 3d⁶ 4s². It can lose electrons from both 4s and 3d orbitals. This allows iron to show +2 (ferrous) and +3 (ferric) states commonly, and also +4, +6, or +1 in some compounds.
Sodium, calcium, and lithium are from s-block elements. They have only one or two valence electrons in s-orbitals. Their fixed oxidation states are +1 for sodium and lithium, and +2 for calcium. They cannot show variable oxidation numbers because they lack d-electrons.
Therefore, among the given options, only iron exhibits variable oxidation numbers.
Answer:Iron (Option C)