I like this kind of question as I dont have to locate the conclusion in the argument. It is present in the question stem.
Board's Conclusion: Local wells that supply drinking water are polluted and causing high cancer rate.
Let's analyze each option by PoE.
Civic Leader: The high cancer rate among our citizens is the result of hazardous material produced at your plant.
Board of Directors: Our statistics show that rates of cancer are high throughout the valley in which the plant is situated because local wells that supply drinking water are polluted, not because of the plant.
Which of the following, if true, most seriously weakens the board’s claims?
(A) The statistics do not differentiate between types of cancer
. [This is Shell Game fallcy. Argument says cancer rate while this choice discusses on tyes of cancers. Incorrect
(B) Nearby communities have not changed the sources of their drinking water. [New info is added but irrelevent/out of scope to our concern. Incorrect
(C) Cancer-causing chemicals used at the plant are discharged into a nearby river and find their way into local wells. [Best choice
. If the chemicals from the plant are discharged into the river which is a source for wells then the plant indeed in causing higher cancer rate.]
(D) The plant both uses and produces chemicals that have been shown to cause cancer [This is rather strengthing the conclusion. Incorrect
(E) Some of the pollutants cited by the board as contaminating the local wells have been present in the wells for decades. [This is rather strengthing the conclusion. Incorrect