Environmental Pollution

Step 1: Understanding the Concept:
Eutrophication is the enrichment of water bodies with nutrients, leading to excessive growth of algae and subsequent oxygen depletion.

Step 2: Detailed Explanation:
Eutrophication is caused by an excess of nutrients, particularly phosphates and nitrates, from fertilizers and sewage. These nutrients cause an algal bloom. When the algae die, their decomposition by bacteria consumes large amounts of dissolved oxygen, creating "dead zones" where aquatic life cannot survive. Therefore, it is characterized by a reduction in dissolved oxygen due to nutrient (phosphate) pollution.

Step 3: Final Answer:
Eutrophication is due to the reduction in dissolved oxygen from phosphate pollution, which corresponds to option (B).