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.