Eutrophication is often the result of
a. increases in animal populations
b. lack of oxygen in the water
c. fertilizer runoff entering aquatic ecosystems
d. a decrease in phytoplankton populations

c. fertilizer runoff entering aquatic ecosystems

Eutrophication is a phenomena which occurs when the water body becomes over-enriched with the minerals and nutrients which get deposited due to surface run off. This results in the excessive growth of algae and other plants, the available oxygen present in the water depletes which results in the mortality of the aquatic animals.

The fertilizers which are used for the purpose of agriculture, are taken away by the water and wind erosion. This water gets added up to the water body. Hence, resulting in eutrophication.  

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.