Iron tablets can be used to supplement the diet. They contain FeSO₄. Which one of the following aqueous reagents could be used under suitable conditions to determined the percentage of iron in the tablets by titration?

Iron supplements contain soluble iron(II) salts to replenish them in the human body in case of deficiency. Iron supplements usually contain about 50mg per tablet of iron(II). Potassium manganate(VII), Mr = 158.034, is a good primary standard, being available to a high degree of purity, having a large relative mass and being stable in the environment thus it is used to determine the percentage of iron in the tablets by titration.