Of the following which is most likely to result after a farmer sprays his crops with pesticides

You forgot the options, but here are some options, maybe you meant those?

1. The crops die.

No, this is not likely. good pesticides don't kill the crops

2. The insect population increases.

No - rather it will decrease

3. The soil dries up and blows away.

Again, not unless the pestiide is really bad

4. A neighbor’s well tests positive for pesticides.

yes, this is what is likely to happen