I^- H5IO6 BrO2 HClO I2

Question

Which could not disproportionate?

Answer 1

To disproportionate, one atom of an element must be oxidized and another atom of the same element must be reduced.
I^- is already at it lowest oxidation state; it can't go lower. not possible.
H5IO6. I is +7 in this compound which is the highest it can go. not possible.
HClO. Cl is +1 here, it can go up as well as down. possible if conditions are right.
etc. A caution here. Just because it is somewhere in the middle of its oxidation state range doesn't mean it WILL disproportionate. Conditions (I look at the E^o values) must be right.