This does not affect the argument. The author is not concerned with whether the water is safe to drink, but with whether the water is palatable (i.e., whether it tastes unacceptably salty).

This strengthens the argument. It reinforces the idea that Albritten’s groundwater will become increasingly salty in the coming decades and thus, will become unpalatable. 400 mg per liter is much higher than 250, which is the concentration at which water becomes unpalatable.

This does not affect the argument. The author is not concerned with why the salting occurs, but with the consequences of the heavy salting.

This weakens the argument. It exploits the author’s assumption that the salt concentration of Albritten’s groundwater has increased because of heavy salting. (D) says the groundwater has always been salty—much saltier than the nearby area that does not heavily salt its roads.

This does not affect the argument. The author only makes an argument about the consequences if Albritten’s roads continue to be heavily salted. The roads not being salted at the current rate is outside the scope of the author’s argument and is not relevant.