Maximum photosynthesis takes place by

The correct option is C Phytoplankton
Primary production is the synthesis of food through the process of photosynthesis, which uses light as its source of energy. The primary production in the ocean is performed by free-living microscopic organisms called phytoplankton. Larger autotrophs, such as the sea grasses and seaweeds are generally confined within the photic zone. The majority of free-floating production takes place within microscopic organisms (phytoplankton)

So, the correct answer is 'Phytoplankton'