Toxicologist: Recent research has shown that dioxin causes cancer in

This is a 'describe the role' question. We start with breaking down the argument into its component parts.

The toxicologist's conclusion is that 'the use of dioxin should be completely banned'. The conclusion is a recommendation.
Why does the toxicologist think so? Because 'Recent research has shown that dioxin causes cancer in rats'. So the fact that 'dioxin causes cancer in rats' is evidence (premises) supporting the main claim

C, D, and E all say that this fact is evidence supporting the claim.

According to C, the claim is that 'similar research will never be done on humans'. But the claim isn't that such research WILL not be done; the expert has recommended that such research SHOULD not be done. So C can be eliminated

E can be eliminated for similar b reasons. According to E, the claim is that 'similar research HAS never been done'. Again, that is not the argument's claim.

Answer choice D, the right answer, will confuse some test takers because of the way it is worded.
"It is presented as a finding that motivates the course of action advocated in the conclusion"

D does not use any word such as 'premises' or 'evidence'. It does not restate the conclusion. But D perfectly describes the role of the dioxin and rat cancer information.
The 'course of action advocated' is that 'such research should not be done'.
The phrase 'finding that motivates the course of action advocated' is a paraphrase of 'evidence that leads to the recommendation'.

Posted from my mobile device