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The stability of +1 oxidation state increases in the sequence
A
Ga < In < Al < Tl
B
Al < Ga < In < Tl
C
Tl < In < Ga < Al
D
In < Tl < Ga < Al
Detailed Solution
Group 13 elements have the configuration $ns^2np^1$ and can show +1 and +3 oxidation states.
Down the group the $ns^2$ electrons become increasingly reluctant to take part in bonding, because the intervening d and f electrons shield the nuclear charge poorly. This is the inert pair effect.
As a result the +1 state becomes more and more stable on going down the group, while the +3 state becomes less stable.
Stability of the +1 state: Al < Ga < In < Tl (for thallium, +1 is the predominant state).
Down the group the $ns^2$ electrons become increasingly reluctant to take part in bonding, because the intervening d and f electrons shield the nuclear charge poorly. This is the inert pair effect.
As a result the +1 state becomes more and more stable on going down the group, while the +3 state becomes less stable.
Stability of the +1 state: Al < Ga < In < Tl (for thallium, +1 is the predominant state).