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# In III A group ,Tl (Thalium) shows stable +1 oxidation state, while boron shows stable +3 oxidation state. It is due to

$(a)\;\text{Presence of lone pair of electrons in II} \\ (b)\;\text{inert pair effect} \\(c)\;\text{large ionic radius of Tl ion} \\(d)\;\text{none of these}$

For TL, after the removal of one electron from p-orbital , the remaining $6s^2$ electrons behave like stable noble gas and do not take part in compound formation.
This reluctance of the s-electron pair to take part in chemical combination is called inert pair effect.
So $+1$ oxidation state more stable for $Tl$
Due to the absence of inert pair effect B does not show +1 oxidation state.
Hence b is the correct answer.