Nutritionist: Vitamins synthesized by chemists are exactly the same as

(A) The author doesn't have to assume that using vitamin pills is waste of money. He's only arguing that buying more expensive vitamin pills is a waste of money (compared to buying the cheaper, seemingly identical stuff)

(B) "always" is too strong. The author is definitely assuming that "sometimes" the natural vitamins are priced higher.

(C) "All" is too strong. If a vitamin pill were 100% natural, that wouldn't hurt the author's argument in any way.

(D) "false advertising" is too out of scope. A vitamin that claims it contains more natural ingredients isn't lying, in the author's eyes. It's just highlighting a feature of the product that the author thinks adds no value, since natural and synthetic vitamins are chemically identical.

(E) In order to weaken this argument (to argue the opposite of the conclusion), we need to justify why it's worth spending more for certain brands even if the vitamin content is exactly the same. This answer, if negated, says that there might be OTHER ingredients that make one brand more worthwhile. Maybe vitamins are harsh on your stomach, but some of the pricier brands include ginger in the vitamin, helping to ease in the digestion of the pill. Maybe some brands have coated capsules that make it easier to swallow the pill.