Actinoids exhibit a wide range of oxidation states due to the involvement of multiple orbitals in bonding.
Unlike lanthanoids, actinoids have:
Partially filled 5f orbitals
Accessible 6d orbitals
Outer 7s electrons
Because the energies of these orbitals are very close, electrons from all three can participate in bonding.
When orbitals of similar energy levels are available, multiple electrons can be lost or shared, leading to variable oxidation states.
Electropositivity — not the main reason.
Large atomic number — not directly related to oxidation states.
Large atomic size — minor factor.
Comparable energies of 5f, 6d, 7s orbitals — correct reason.