2. In polar regions, phytoplankton. a thrive all year round, because nutrients are always plentiful
b. thrive during the summer, when sunlight increases
c. cannot survive, because the waters are too cold
d. cannot survive, because a permanent thermocline traps nutrients deep below the surface

b. thrive during the summer, when sunlight increases

Explanation:

Phytoplankton production is highly associated with latitude and season. In polar regions, phytoplankton production is reduced to the summer (and eventually spring) season. In consequence, the zooplankton that eats phytoplankton only can survive during the spring and summer seasons. On the other hand, in tropical areas, there exists a nearly constant phytoplankton production all year round.  

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