A, the smaller the solubility product with similar chemical formula, the easier it is to form precipitation, and KSP (mco3) > KSP (RCO3), so it is RCO3 precipitation, so A is correct;

B, test Fe3+ should be added to KSCN solution. If the solution turns red, it proves that Fe3+ exists, and chlorine water can oxidize Fe2+ into Fe3+, so chlorine water cannot be added, so B is wrong;

C, silver ions in the solution will also generate white precipitate, so the operation of detecting SO42- is as follows: firstly, add hydrochloric acid to eliminate interference, and then add barium chloride solution to generate white precipitate, and this solution must contain SO42-, so C is wrong;

D, carbonate ion is hydrolyzed in solution, and CO32-+H2O? HCO3-+OH-, the amount of anionic substances in the solution becomes larger, so D is wrong;

So choose a.