The stimulus actually is redefining the term "natural." The argument begins by talking about "natural" in how it relates to "organic foods." Then, it talks about how all plants turn molecules into "natural" compounds. Then it says that all compounds come from nature, so their all "natural." These are all different takes on what it means to be "natural," thus the argument is redefining the term in its favor.
(C) is incorrect because the argument doesn't appeal to, or even mention, scientific methods. Rather, it talks about things that sort-of feel "sciency" (compounds, molecules, etc.) but this is not related to scientific method.