The correct option is A Cyanobacteria
Cyanobacteria often called the "blue -green algae" are unicellular, aquatic, photosynthetic bacteria, which often grow in colonies large enough to be visible to the naked eyes. They are the oldest known fossils, dating to more than 3.5 billion years. They have facilitated the oxygenic atmosphere we thrive in. Some of them are excellent nitrogen fixers for rice and bean cultivation.
Hence, the correct answer is option (A) - 'Cyanobacteria'.