This question is asking you to find the "strongest objection" to the reasoning in the argument. Given the phrasing of the question, this means (1) more than one answer could provide an objection to the argument but you're looking for the best one and (2) there should be a natural failing to the argument that you want to identify.
So, before turning to the answer choices, see if you can identify a gap in the logic of the argument. I see one in the conclusion. It states that high levels of galactose must be carcinogenic, but two things were discussed in the argument: (1) levels of galactose and (2) an enzyme in the body. The argument has not ruled out that the enzyme is involved, rather than the galactose.
Answer Choice (D) is correct because it spots this gap in the logic. The argument says that a low level of this enzyme means the body can't process the galactose but then just goes on to assume that this means the galactose is bad. There is just as good a chance that the low level of the enzyme is involved with cancer.
Answer (C) is incorrect because it discusses things that are outside the scope of the argument. This argument only focuses on the enzyme and the galactose, there is no need to bring in a discussion of other substances. Additionally, the statement that "it is well known that there are many such substances" is outside knowledge, never discussed in the stimulus. You should always be wary of answer choices which bring in outside information.
