What type of organisms was most likely feeding on this sample ?

I think Number C is right because Since the organism eukaryotic we can safely assume that it is neither an archaea nor a bacteria since they are both prokaryotes. Secondly, since it can not photosynthesize, it can't be a plant because all plants can photosynthesize. The presence of a cell wall tells us that it can't be an animal cell since they lack cell walls. Therefore, the organism found is a fungus-like protist. Fungi are prokaryotic cells that are unable to photosynthesize and they have cell walls.