Which of the following does not lead to an increase in phytoplankton reproduction?
a. increased nutrient availability
b. increased sunlight
c. increased oxygen levels
d. increased levels of carbon dioxide

The answer as to which of the following does not lead to an increase in phytoplankton reproduction would be C. increased oxygen levels.

Phytoplankton are organisms that are usually called as "single-celled plant", but this is a misnomer or a wrong indication or usage of name thus it is discouraged to term them as such. They can do photosynthesis which gives them energy and food and allows them to release oxygen which isn't needed by them. They need carbon dioxide and sunlight in order to do photosynthesis, and the nutrient availability would be an extra factor on their growth.