Transition metals have variable oxidation states because they haveAmore than one electron in the outermost shellBan electron octet in their penultimate shellsCstable penultimate shellsDunstable penultimate shells

Transition metals have variable oxidation states because they have more than one electron in the outermost shell (A).

Here's why:

Transition metals have partially filled d orbitals. The d and s orbitals in the outermost and penultimate energy levels can lose electrons, leading to multiple oxidation states. This is because the energy difference between these orbitals is small, so both can be involved in bond formation.

B. Having an electron octet in their penultimate shells does not explain the variable oxidation states of transition metals.

C. Having stable penultimate shells does not explain the variable oxidation states of transition metals.

D. Having unstable penultimate shells does not explain the variable oxidation states of transition metals.