The species among the following which can act as an acid only.

The species which can accept as well as donate H+ can act both as an acid and as a base.HSO4-base + H+⇌  H2SO4HSO4-acid ⇌ SO42-+ H+   ,SO4-2can accept proton,so acts as only base, H3 O+can donate proton, so acts as only acid Cl-can accept proton,so acts as only base