The bond dissociation energy of the $E-H$ bond in the $H_2E$ hydrides of group $16$ elements (where $E = O, S, Se, Te$) follows which order?

Rhombic sulphur has the following structure:

All the elements of the oxygen family are

How many total statements are correct in the following?
$(i)$ $SO_2$ - antichlor,preservative
$(ii)$ $O_3$ - Disinfectant
$(iii)$ $Cl_2$ - Antiseptic
$(iv)$ $O_2$ - used in oxy-acetylene welding
$(v)$ $PH_3$ - used in smoke screens
$(vi)$ $HNO_3$ - Pickling of stainless steel

The catalyst used in the manufacture of $H_2SO_4$ by the contact process is

The correct order of $K_a$ in aqueous solution at $298 \ K$ for the hydrides of group $16$ elements is: