The correct option is A Bacteria
Detritus is that waste material left behind from other biological processes. Microorganisms (such as bacteria or fungi) break down detritus, and this microorganism-rich material is eaten by invertebrates, which are in turn eaten by vertebrates. Many freshwater streams have detritus rather than living plants as their energy base. This completes the food chain based on detritus. This detritus cycle plays a large part in the purification process, whereby organic materials carried in by rivers is broken down.