@amhuynh I think it is confusing because the "When" in sentence 2 signals a sufficient condition is about to be stated, "over nutrification of estuary waters" and thereby "algae proliferates" is the necessary condition. If you take the contrapositive, it looks very similar to (D).

3/3 on my actual take, and 1/3 on my blind review. This strategy is making me overcomplicate things