I would say
A. Living things decay
Because when living things break down/decay, it becomes a nutrient for the soil like a fertilizer. B is wrong because although plants is a living thing, humans is also, but they don’t go through photosynthesis, which is transforming sunlight into food. C is wrong because runoff enters the nearby bodies of water, this is just water coming down (like raining, snowing, hailing) somewhere into the runoff to go into the nearby bodies of water and goes over the same water cycle over and over again. Although bacteria does convert carbon in the air (D) and form the plant host can use, but it’s part of the ground soil, so it’s not D.