Carbonic acid is a polyprotic acid. write balanced chemical equations for the sequence of reactions that carbonic acid can undergo when it's dissolved in water.

Carbonic acid (H₂CO₃) dissociation in water takes place in 2 steps. its a polypro tic acid as it has 2 H⁺ ions it can donate.
the dissociation reactions in water are as follows;
H₂CO₃  + H₂O ---> HCO₃⁻ + H₃O⁺
in the first dissociation it releases a proton that combines with water to form a hydronium ion.
HCO₃⁻ + H₂O  > CO₃²⁻ + H₃O⁺
second dissociation results in the carbonate ion and hydronium ion formation.
As the ratio of acid dissociation constant ka1/ka2 > 10⁴ , dissociation takes place in 2 steps. 

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged

The answer is d for this question