Supplements of the water-soluble vitamins ______ and _______ are most likely to cause toxicity symptoms. A.thiamin; riboflavin B.vitamin B-12; pantothenic acid C.vitamin B-6; vitamin C D.biotin; vitamin B-12 Solution Of the different water soluble vitamins, four i.e. folate, niacin, vitamin B6 and vitamin C have shown the potential for toxicity. Hence the correct answer is option C. – vitamin B6 and vitamin C..