Answer:
HCO3-
Step-by-step explanation:
The Bronsted-Lowry acid-base theory states that in a chemical equation, acid is represented by the element that is donating protons within the equation, while the base is the substance that is receiving these protons.
In the case of the chemical equation HCO3− + H2S → H2CO3 + HS−, the element HCO3− is the base, since it is this element that receives readiness that transforms it into H2CO3, while the element H2S is the acid since it is that element that donates protons turning into HS−.