The answer is d) none of these
In a redox reaction, a reducing agent loses electrons and an oxidizing agent gains electrons.
This means that stronger reducing agents will lose electrons more easily than weaker reducing agents.
In the case of hydracids, oxidation can only remove an electron from the halogen ion, which means that the stability of the halogen atom will determine how strong the reducing ability of the hydracid will be.
In other words, the easier it is to remove an electron from a halogen ion, the stronger reducing agent the hydracid will be.
Because the size of the ions increases down a group, the ions with the largest ionic size will tend to lose an electron more easily.
This implies that hydriodic acid,
As a result, electrons are more difficult to remove from
Final answer, d) none of these.