The correct answer is A. Oxalic acid.
Oxalic acid is a strong dicarboxylic acid that is used in a variety of applications, including cleaning, metalworking, and tanning. It is also used as a food additive and in the production of pharmaceuticals.
Oxalic acid is effective in removing stains of iron compounds because it is a chelating agent. Chelating agents are molecules that can form complexes with metal ions. In the case of oxalic acid, the chelate complex is formed between the oxalic acid molecule and the iron ion. This complex is then easily removed from the surface, taking the iron stain with it.
Benzoic acid, phthalic acid, and cinnamic acid are all organic compounds, but they are not used to remove stains of iron compounds. Benzoic acid is used as a preservative and in the production of pharmaceuticals. Phthalic acid is used in the production of plastics and resins. Cinnamic acid is used in the production of perfumes and flavorings.
Oxalic acid is the only organic compound listed that is used to remove stains of iron compounds.