The relative stability of +1 oxidation state of group 13 elements follows the order

Strategy: Apply the inert pair effect to rank the stability of +1 oxidation state across Group 13 elements.

Step 1: Inert Pair Effect Down the Group Going down Group 13, the $ns^2$ pair becomes less available for bonding due to poor shielding and relativistic effects (especially heavy elements). The +1 oxidation state (retaining the $ns^2$ pair) becomes progressively more stable going down.

Step 2: Ordering

• Al: +1 state very unstable (strongly prefers +3)
• Ga: Slightly more stable +1 than Al
• In: More stable +1 than Ga
• Tl: Most stable +1 in the group (Tl⁺ is its predominant state)

$\ce{Al < Ga < In < Tl}$

$\boxed{\text{Answer: (D) }\ce{Al < Ga < In < Tl}}$