Ah I chose C... but D is definitely the obvious answer jumping out at me... because of the subset of rats... 60% were exposed... and since it was a subset it's smaller etc etc blah blah

say 100 rats dies due to liver damage.
so according to the stimulus- 90 rats were not exposed to envi toxins.
the 10 rats which were exposed to toxin constitute 65% of the rats exposed to envi toxins ie approx 15 rats were exposed.
so we know the total number of rats studied was greater than 100.

Now lets look at options.
(A) Environmental and nonenvironmental causes of liver disease in rats are mutually exclusive. [causes is wrong. we have checked for only one envi toxin. so we cannot generalize. Eliminated!]

(B) There is only one cause of fatal liver disease in rats. [we dont know. we are not given info about it. we have stimulus dealing with just one cause. that does not mean there is only one cause. Eliminated!]

(C) Environmental toxins are not particularly dangerous to the livers of rats. [if they are not dangerous then why will 65% of the rats exposed to them die? Eliminated!]

(D) Only a small portion of the entire group of rats studied was exposed to environmental sulfur dioxide. [Bingo. 15 out of a number greater than 100 is a small portion. Correct!]

(E) Most rats will not suffer from exposure to low levels of sulfur dioxide [how can we talk about most rats when we ourselves in the experiment subject only 15 rats out of more than 100 rats?? Eliminated!]

Hope this helps!