Answer
Procedure
To determine the correct reaction we will need to recall the Brønsted-Lowry theory of acids and bases, which indicates that a Brønsted-Lowry acid is any species that can donate a proton, and a base is any species that can accept a proton. Additionally, a conjugate acid is the species formed after the base accepts a proton. By contrast, a conjugate base is the species formed after an acid donates its proton.
Based on this theory, for the reaction:
H₂CO₃ + H₂O → H₃O⁺ + HCO₃⁻
We will have that H₂CO₃ donate a proton hence acting as an acid and H₂O receives a proton, having the role of a base. Based on the previous, the conjugate base will be HCO₃⁻ and the conjugate acid H₃O⁺.
Therefore the correct answer is