If a pickle were an animal, what wouild be the function of the exterior surface

The function of the exterior surface would be to prevent predation.

Most pickle are made from the cucumber. The outer layer of cucumber has substance that cause a bitter taste and cytotoxic effect that could harm the predator. The harmful effect will make the predator hesitate to eat the cucumber when they could find other food. It might also kill the predator and prevent their number growing from eating the pickle.

The correct answer is d. Oxygen

Explanation:

Phytoplanktons are responsible for the fixation of approximately half of the global carbon therefore seawater has high CO2 concentration which is required by phytoplanktons to make their food.  

They are the primary producers of oceans and they are responsible to support the food chain of oceans. Factors that can limit their growth are mainly sunlight and nutrients like phosphorus, nitrogen, etc.

As phytoplanktons are photosynthetic they release oxygen itself as a byproduct therefore oxygen is not a limiting factor to phytoplanktons. So the right answer is d.