They are calling the main conclusion to the point that the children will get cured once they take the supplement the company is preparing
So i think that's why D is correct because unless children agree to the fact to intake the investigation, the claim made by the pharma company cannot be considered suitable
However i felt C because C proves the point that unless there is a deficiency of iron in the body , there is no need to take any external supplement but that is not their main conclusion, main conclusion is to prove that the claim made by the company is correct , to which D seems to be a better assumption