Final answer:
Humus is the correct answer, as it is an organic substance in soil that benefits plants and can also be consumed with pita.
Step-by-step explanation:
The organic substance in soil formed from decaying matter, which is beneficial for plants and also a food item commonly served with pita, is humus (Option A). Humus refers to the component of soil composed of decomposed plants and animals, as well as living and non-living microorganisms. It significantly improves the soil structure by enhancing its water and mineral content, which in turn supports plant life. On the other hand, peat (Option B) is an incompletely decomposed organic matter found in bogs, often used as fuel or a soil additive, while clay (Option C) and loam (Option D) refer to specific types of soil particles and soil mixtures, respectively. Therefore, the correct answer for the substance that is both part of the soil and eaten with pita is humus.