The correct answer is A. Oxalic acid.
Oxalic acid is a strong acid that can dissolve rust. It is available in both liquid and solid form. To use oxalic acid to remove rust stains from clothes, you will need to mix it with water according to the instructions on the product label. Then, apply the mixture to the stain and let it sit for the amount of time specified on the label. Rinse the mixture away with water and then wash the clothes as usual.
Petrol, alcohol, and H2O2 are not effective at removing rust stains from clothes. Petrol is a flammable liquid that can be dangerous to use around clothing. Alcohol is a solvent that can damage some fabrics. H2O2 is a bleaching agent that can also damage some fabrics.
Oxalic acid is the safest and most effective way to remove rust stains from clothes. It is important to follow the instructions on the product label carefully to avoid damaging your clothes.