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

Question

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