Which of the following can act both as Bronsted acid and as Bronsted base?

(A) ${\mathrm{Cl}}^{-}$
(B) ${\mathrm{HCO}}_3^{-}$
(C) ${\mathrm{H}}_3{\mathrm{O}}^{+}$
(D) ${\mathrm{OH}}^{-}$

Those substances that accept protons are called Bronsted bases and those that donate protons are called Bronsted acids.

${\mathrm{HCO}}_3^{-} + {\mathrm{H}}^{+} \leftrightharpoons {\mathrm{H}}_2{\mathrm{CO}}_3$ Bronsted base

${\mathrm{HCO}}_3^{-} \leftrightharpoons {\mathrm{H}}^{+} + {\mathrm{CO}}_3^{- -}$ Bronsted acid

${\mathrm{HO}}^{-} + {\mathrm{H}}^{+} \leftrightharpoons {\mathrm{H}}_2\mathrm{O}$ ---Bronsted base

${\mathrm{HO}}^{-} \leftrightharpoons {\mathrm{H}}^{+} + {\mathrm{O}}^{-2}$----Bronsted acid