The right answer is A. producer.
Phytoplankton are all cyanobacteria and microalgae (microscopic plants) present in surface waters and drifting with the currents. Unknown because invisible to the naked eye, phytoplankton is the lung of our planet. Thanks to photosynthesis, it produces more than half of the Earth's oxygen and consumes half of the carbon dioxide. Phytoplankton is essential for marine life because it is also at the base of the ocean food chain.