We can paraphrase the question stem as : so what which of the below would help us believe more in the claim of the toxicologists that the set standard i.e. 1/2 gallon of drinking water is not enough to protect the public, given:
1. industrial solvents (IS) are harmful
2. IS often drain into public water
3. current tests provided a threshold that if the exposure is to drinking 1/2 gallons of water daily - it is safe

One possible way, I thought, was to invalidate the setting of the tests conducted, from which this threshold was derived i.e. to say something like actual concentration of IS is 2x the concentration used during the test.
Now lets look at the options:
(A) The figure of one-half gallon a day includes water contained in beverages such as soft drinks, which are often bottled at locations distant from where they are consumed
=> this acts more like a weakener for the toxicologists claim, stating the figure provided is all-inclusive, thus makes us believe more in the threshold provided and less in the toxicologists claim

(B) Some industrial solvents have less toxic but more expensive analogues that industry has not adopted for use.
=> Out of scope

(C) Water treatment centers usually filter out bacteria and other organisms before the water is pumped into public supply systems.
=> Out of scope, doesnt impact the conclusion i.e. this might already be in practice and the scenario mentioned in PA is present despite this

(D) Industrial polluters are rarely fined or punished, even when they knowingly allow toxic chemicals to enter water supplies.
=> Out of scope

(E) More solvent enters the body through skin absorption, during washing and bathing, than through drinking.
=> Yes, this gives an alternate source of entry not factored in the threshold given for drinking water