Concept:Eutrophication is the nutrient-driven overgrowth of algae, which eventually depletes oxygen and harms aquatic life.
Explanation:Eutrophication starts when excess nutrients, such as nitrates, enter a water body from fertilisers or sewage.
These nutrients trigger massive algal growth, often called an algal bloom.
When the algae die, decomposers break them down and use up large amounts of dissolved oxygen.
As oxygen levels fall, aquatic organisms like fish cannot survive and die.
Therefore, the correct sequence is: excessive nitrates → massive algal growth → lack of oxygen → death of aquatic organisms.
Answer:Option B is correct.