Which compound can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base?

a. water

b. oxygen

c. sodium hydroxide

d. hydrochloric acid

Water.

In certain circumstances, water acts as a proton donor.

e. g NH3 (aq) + H2O (l) - - > NH4 + (aq) + OH - (aq)

In other circumstances, water can act as a proton acceptor.

e. g HNO3 (aq) + H2O (l) - - > H3O + (aq) + NO3 - (aq)