Many plant parasitic nematodes feed on moist tissues of plants, therefore A is incorrect.
Many parasitic nematodes also create wounds on the roots of plants while feeding, making them vulnerable for other parasits. Therefore, B is incorrect as well.
The parasitic nematodes are the ones that feed on animals and plants, because a parasitic relationship means that the nematode needs the animal/plant to survive and reproduce, and will cause harm to them as well. Therefore, C is incorrect too because when nematodes are parasites in animals they will feed from theirs fluids.
The nematodes that eat bacteria in the air are free-living nematodes, therefore the correct answer is D. the bacteria in the air