Which of the following halides is least stable and has doubtful existence ?

The bottom elements of group 14 show the inert pair effect. Due to this inert pair effect, the oxidation state of the bottom elements reduces by 2. Therefore, the bottom elements of group 14 show a stable +2 oxidation state. In the case of dihalides of group 14 elements, the oxidation states of the group 14 elements are +2, which is stable for a bottom element like lead. Therefore, in the case of tetrahalide lead tetra halide is very unstable and has doubtful existence.