I don’t have my notes on me but I’m assuming this questions has to do with the H+ concentration in each compound. The more H+ molecules in a solution, the more acidic it is, the lower it’s pH is. So for the low pH of 4 (most acidic), H2CO3 is the most concentrated since there are the highest number of H+ ions per molecule. For the medium pH of 7 (neutral), HCO3- is most concentrated. For the high pH of 10 (basic), CO3^2- is most concentrated because there are the least number of H+ ions, in this case, none at all.
