Given the equation representing a reversible reaction: which formula represents the h+ acceptor in the forward reaction?

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Option A: HCO3-(aq.)

Reason:
From the reaction, it can be seen that following reaction occurs in forward direct

HCO3-(aq)     +     H2O(l)      →      H2CO3(aq)     +      OH-(aq)

In above forward reaction, HCO3- accepts proton from H2O to generate H2CO3. Thus, according to Lowry and Bronsted theory of acid-base, HCO3- is a base, while H2CO3 is a conjugate acid.