In the iodination of benzene, which compound causes the reduction of iodobenzene back to benzene?

Hydrogen iodide

Hydrogen iodide (HI) is a strong reducing agent, and it can reduce iodobenzene back to benzene. So the answer is (3).

The other compounds listed are not reducing agents, so they would not cause the reduction of iodobenzene back to benzene.

Mercuric oxide is a catalyst that is used in the iodination of benzene, and iodic acid is a product of the reaction. Nitric acid is an oxidizing agent, and it would not cause the reduction of iodobenzene back to benzene.