Answer:
The activity that would effectivly keep the oxygen cycle stable would be replanting treesin a deforest area, because now not only are you recreating that area for organisms to live in again, but creating more oxygen.
A. Dumping waste wouldn't help resolve anything, because now you are ruining not only the environment but the water supply, and the aquatic organisms.
C. Raising more cows on farm, wouldn't really resolve anything much.
D. Pruining trees would basically be cutting them, which wouldn't help with oxygen levels, moreover, you are decreasing it.
