Which of the following is considered a major source of water pollution?

Sediment is indeed a major source of water pollution, particularly in freshwater ecosystems. When soil and other particulate matter are washed into water bodies due to erosion, construction activities, or deforestation, they can cloud the water, reducing sunlight penetration and affecting photosynthesis in aquatic plants. Furthermore, sediment can carry with it nutrients, pesticides, and other pollutants that can harm aquatic life, disrupt ecosystems, and affect water quality. Excess sediment can also lead to issues such as habitat degradation for fish and other organisms, making it a significant contributor to water pollution.

The other options, while also serious environmental concerns, represent specific types of pollutants or pollution incidents rather than ongoing sources of water pollution like sediment. For instance, plastic waste and oil spills are indeed harmful to aquatic environments but are generally considered more specific events or materials rather than continuous contributors to the accumulation of pollutants in water systems. Chemicals can vary widely in their sources and impacts, but sediment's role in the transportation and dispersion of various pollutants ultimately makes it a fundamental and pervasive source of water quality issues.