The anhydrides of oxyacids are
(i) $\mathrm{HClO}_4 \rightarrow \mathrm{Cl}_2 \mathrm{O}_7$
(ii) $\mathrm{HClO}_3 \rightarrow \mathrm{Cl}_2 \mathrm{O}_6$
(iii) $\mathrm{HClO}_2 \rightarrow \mathrm{ClO}_2$
(iv) $\mathrm{HOCl} \rightarrow \mathrm{Cl}_2 \mathrm{O}$
The correct answer is $\mathrm{Cl}_2 \mathrm{O}$ i.e. option (b).