Thanks for the question! The problem with (B) actually isn’t that it weakens Gilbert’s argument but doesn’t strengthen Sabina’s. The problem with (B) is actually that it’s irrelevant to the topic being discussed. Let’s look at the argument carefully. Gilbert is talking bout the ingredients in the cookies and claiming that there are artificial ingredients. Sabina says that no, the ingredients he thinks are artificial are actually natural. (B) tells us that not all chemicals that are part of the manufacturing process are ingredients of the cookies. That doesn’t tell us anything specifically about what’s at hand, since we know for sure that these alphahydroxy acids are ingredients of the cookies. (B) just tells us that there are some things that the plant produces that aren’t, which doesn’t really help either party.
Hope this helps! Feel free to ask any other questions that you might have.