Eutrophication is caused by
a. an increase in nutrients
b. a decrease in nutrients
c. the introduction of new species of animals
d. the introduction of new species of plants

An increase in nutrients.

Explanation:

Eutrophication is known as the natural ageing of the lake. The lake gets covered with the excess algae and microorganisms that contains the algal bloom in the lake.

The eutrophication mainly occurs because of excess phosphorus and nitrogen containing components. The increase in the nutrient enrichment also causes the oxygen depletion in the lake. This also leads to the increase of phytoplankton  in the body.

Thus, the correct answer is option (a).

does it have options?