The correct answer is D) toxicity.
Taking too much of a specific vitamin or mineral can lead to toxicity, which can have negative effects on the body. In this case, Samantha noticed that three of her vitamins were extremely high in Vitamin A, and if she continued taking them at those levels, it could lead to Vitamin A toxicity. Therefore, she made the right decision by stopping taking these vitamins to prevent developing any health concerns due to excessive intake of Vitamin A.