To solve this problem, we need to identify which oxoacid of sulfur contains a $-O-O-$ linkage.$\newline$Let's analyze each option:$\newline$1. $H_2SO_4$ (sulphuric acid):$\newline$• The structure of sulphuric acid is $O=S(OH)_2$ with no $-O-O-$ linkage.$\newline$2. $H_2S_2O_8$ (peroxodisulphuric acid):$\newline$• The structure of peroxodisulphuric acid is $HO_3S-O-O-SO_3H. \newline$• This contains a $-O-O-$ (peroxy) linkage between the two sulfur atoms.$\newline$3. $H_2S_2O_7$ (pyrosulphuric acid):$\newline$• The structure of pyrosulphuric acid is $HO_3S-O-SO_3H. \newline$• There is no $-O-O-$ linkage present.$\newline$4. $H_2SO_3$ (sulphurous acid):$\newline$• The structure of sulphurous acid is $O=S(OH)_2$ with no $-O-O-$ linkage.$\newline$From the analysis, only peroxodisulphuric acid $(H_2S_2O_8)$ contains the $-O-O-$ linkage.$\newline$Therefore, the correct option is Option 2.