How many of the following contains a peroxy linkage?

Hydrogen peroxide${\mathrm{K}}_3{\mathrm{CrO}}_8$ , chromic acid, permanganic acid, perxenic acid, pernitric acid, ${\mathrm{H}}_3{\mathrm{PO}}_5$ , Marshall's acid, Caro's acid, oleum.

The linkage of two oxygen atom is called the proxy linkage. In hydrogen peroxide(${\mathrm{H}}_2{\mathrm{O}}_2$), Potassium tetraperoxochromate.${\mathrm{K}}_3{\mathrm{CrO}}_8$, pernitric acid, ${\mathrm{H}}_3{\mathrm{PO}}_5$ Marshall's and Caro's acid consist the peroxy linkages. 

Thus, the correct answer is 6.