Aluminium is usually found in +3 oxidation state. In contrast, thallium exists in +1 and +3 oxidation states. This is due to:

We first recall the electronic configuration of the relevant elements. For aluminium, which has atomic number 13, the ground-state configuration is $$\mathrm{[Ne]\;3s^{2}\,3p^{1}}.$$ For thallium, atomic number 81, the configuration can be written in a short form as $$\mathrm{[Xe]\;4f^{14}\,5d^{10}\,6s^{2}\,6p^{1}}.$$

Both elements are members of group 13, so each possesses three electrons outside the noble-gas core. Ordinarily, all three outer electrons would be available for bonding, leading to the oxidation state $$+3.$$ This is indeed what we observe for aluminium: the two $$3s$$ electrons and the one $$3p$$ electron are all ionised or shared, giving compounds such as $$\mathrm{Al^{3+}}$$ or $$\mathrm{AlCl_{3}}.$$

Now we look at thallium. Thallium’s outer electrons are $$6s^{2}\,6p^{1}.$$ Experimentally we find that thallium forms many compounds in the $$+1$$ state, for example $$\mathrm{Tl^{+}}, \; TlCl,$$ etc., as well as some in the $$+3$$ state like $$\mathrm{TlCl_{3}}.$$ The key point is that, in thallium, the pair of $$6s$$ electrons tends to remain non-bonding or “inert,” while only the single $$6p$$ electron participates in bonding, giving the oxidation state $$+1.$$

The tendency of the outermost $$ns^{2}$$ electron pair to resist participation in bonding as we move down a group in the p-block is known as the inert pair effect. This effect increases with atomic number because:

Greater nuclear charge $$\;+\;$$ poor shielding by intervening $$d \text{ and } f$$ electrons $$\;\Longrightarrow\;$$ stronger attraction of the $$ns^{2}$$ pair to the nucleus $$.$$

Hence, the $$6s^{2}$$ pair in thallium is held more tightly and is less easily ionised than the $$3s^{2}$$ pair in aluminium. As a consequence, aluminium shows almost exclusively the $$+3$$ state, whereas thallium comfortably exhibits both $$+1$$ (with the inert pair remaining) and $$+3$$ (when all three electrons are removed).

Among the options given, this explanation corresponds to the inert pair effect and not to diagonal relationship, lattice effect, or lanthanoid contraction.

Hence, the correct answer is Option D.