The author is arguing that dioxin is NOT the cause of the reproductive abnormalities because when the dioxin-producing mill shuts down, fish recover relatively quickly even though dioxin is slow to decompose. (The author must think something else in the mill is causing the abnormalities although he doesn't specify what).

We have to weaken the argument that dioxin is NOT the cause of the abnormalities. So, we should look for a choice that suggests to us that dioxin IS the cause.

Choice C matches this prediction. It tells us that dioxin is ejected from the local environment in a few hours, which, in this context, definitely counts as "relatively quickly". So: the mill shuts down, dioxin disappears, and the fish get better. Sounds like dioxin may well be the cause!

(Because this choice establishes a correlation between dioxin's disappearance and the fishes' recovery, it opens up the possibility that dioxin IS the cause, thereby weakening the argument that it isn't. Remember, to weaken an argument we need only find a fact that renders the conclusion less likely to come true--we don't need to completley refute the argument.)