Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts. But why does preparation of aryl iodides requires presence of an oxidising agent ?

Why can aryl halides not be prepared by reaction of phenol with HCl in the presence of ZnCl_2 ?

Aryl bromides can be prepared by reacting silver aromatic acids with Br_2 in Carbon tetrachloride. This reaction is Hunsdiecker reaction/ Balz- Schiemann reaction.

A non-metal A which is the largest consituent of air, when heated with H_2 in 1:3 ratio in the presence of catalyst gives a gas B. on heating with O_2 it gives an oxide C. If this oxide is passed into water in the presence of air it gives an acid D which acts as a strong oxidising agent. To which group of periodic table does this non-metal belong?