Answer: The correct option is A) Oxygen.

Phytoplanktons are single celled microscopic organisms, which are mainly present in the euphotic ( the superficial water surface where sunlight pentrates) zone of the aquatic ecosystem. These are photosynthetic organisms as they absorb sunlight and prepare food through photosynthesis. During this process, they utilize CO₂ and H₂O in the presence of sunlight and produce food and Oxygen as a byproduct.

By doing so, they form the base of aquatic food chain as all the living organisms utilize these byproducts synthesized by them. This helps in the survival and maintenance of the life forms.