A – Wrong
We cannot be sure whether the nematode worms cause the decrease in fertiliser residues or the decrease in fertiliser residues causes the increase in nematode worms. We cannot create causal link between the two due to lack of sufficient information.
B – Correct
Option B just states that nematode worms and fertiliser residues are negatively correlated. It does not make a causal link and just states what the text tries to say. So this is correct.
C – Wrong
Again, with the same logic as A, we cannot infer a causal link between the two things.
D – Wrong
The text does not say anything about fertiliser residues interfering with reproduction of the nematode worms. This is definitely wrong. (Please understand that anything not written in the text will most likely be wrong).