Ans.
The first organism to produce oxygen into the atmosphere were blue-green algae or cyanobacteria that evolved around 3.5 billion years ago. They have green pigments or chlorophyll molecules that help them to perform photosynthesis.
During photosynthesis, they make their food by using carbon dioxide and release oxygen gas into atmosphere as byproduct.
Thus, the correct answer is option). 'cyanobacteria.'
