Answer:
Cyanobacteria.
Step-by-step explanation:
Cyanobacteria are simple, prokaryotic organisms that belong to kingdom Eubacteria. These bacteria are able to perform photosynthesis to get their energy and to produce oxygen.
They are also known as blue-green algae, due to their color. They are most primitive photosynthetic organisms.
Thus, the correct answer is 'cyanobacteria.'