(i) ion is more stable among all, because as we know that the Br atom is an electron withdrawing atom, although it is having loan pair of electrons, so that the positive charge present on the carbon atom is stabilised by the loan pair of the Bromine atom by resonance, but the for the halogens we know that the inductive effect dominates over the resonance effect, hence the Bromine atom will destabilise the positive charge on the carbon atom.

And as has three bromine atoms, the positive charge is more destabilised as compared to others which have two bromine atom, one bromine atom and no bromine atom.

(ii) As we know that Cl atom is more electron withdrawing atom, and also it has vacant d-orbital, so the negative charge on the C, is easily stabilised by the Cl atoms, more is the number of Cl atom attached to the Carbanion, more is the stability of the carbanion.

Hence, CCl₃^-is most stable.