LOL. I love the back and forth on this thread. OP went all gangsta and brought functions in.
My two cents:
Stimulus says there is A LOT more supply than we are currently using. Therefore, it is not the case that we have to change population growth in near future to prevent water shortage.
To weaken this, we need to disturb the connection between stimulus and conclusion.
A) Who cares? Doesn't affect connection w/ water. Eliminate.
B) Water and population connection is clearly present. Leave for now.
C) Same as B. Leave for now.
D) Eventually is compatible w/ argument which narrowed its conclusion to the "near future." Eliminate.
E) Who cares? Doesn't affect connection w/ population. Eliminate.
Left with B and C.
B does weaken the argument. It implicitly allows for the stimulus about supply, but brings up a new issue. Looking back, the stimulus is about Earth's supply of fresh water, that is, its TOTAL fresh water supply. B is bringing up the idea that there might be "significant" variation across regions which brings up the possibility of some regions having less supply than others. Granted, we don't know how those regions' populations compare relative to other regions, but, the mere fact that this supply issue has been demonstrated to not be all we should concern ourselves about weakens the stimulus' connection to the argument.
Looking at C again, the argument is totally fine with it because the argument implies that there is no water crisis coming in the near future and, so the author of argument would say, there is no need for all of Earth's population to adopt conservation methods. Eliminate.
OP: you bring up a lot of interesting thoughts but, ultimately, you are misguided. The LSAT is testing for a very specific type of logical reasoning. You may want to consider going through a Cambridge packet and looking at other strengthen and weaken questions to see more of these same types of questions. The LSAC is testing you. You are not testing the LSAC. Their logic trumps yours as far as this test is concerned. Get with it homie and these answers become a lot more palatable.