What type of organisms are included in the kingdom Eubacteria?

The kingdom Eubacteria consists primarily of single-celled prokaryotic organisms, making this the correct choice. Prokaryotes, which include bacteria, are characterized by their lack of a defined nucleus and membrane-bound organelles. Eubacteria are notable for their diverse metabolic capabilities, which allow them to thrive in a variety of environments. They can be found in soil, water, and within other organisms, playing essential roles in ecosystems, including nutrient cycling and the decomposition of organic matter.

In contrast, the other answer choices represent different types of organisms not classified within Eubacteria. Multicellular eukaryotic organisms are found in kingdoms such as Animalia and Plantae, which are distinctly different due to their complex cellular structure and organization. Heterotrophic polyps, like those found in coral, belong to the phylum Cnidaria and are also eukaryotic. Lastly, the term "photosynthetic organisms" can refer to various types of organisms, including plants and some algae, which are also eukaryotic and not part of the Eubacteria kingdom. Thus, recognizing Eubacteria as prokaryotic is key to understanding its classification.