The correct answer is option 'B': Sanitary landfills.

Sanitary landfills are engineered facilities designed to dispose of solid waste in a controlled manner. They are designed to minimize the impact on the environment and protect public health. One of the major concerns associated with sanitary landfills is the generation of leachate.

Leachate is a liquid that forms when water, often from rainfall, percolates through the waste materials in a landfill. It picks up various contaminants from the waste, including dissolved organic compounds, heavy metals, and other pollutants. Leachate can be highly toxic and pose a significant threat to groundwater and surface water quality if not properly managed.

Below are the reasons why the leachate is an effluent from sanitary landfills:

1. Formation of Leachate:
- When solid waste is disposed of in a sanitary landfill, it undergoes decomposition and creates a liquid called leachate.
- The decomposition process is primarily carried out by microorganisms that break down the organic waste, releasing water and other byproducts.
- As this liquid percolates through the waste, it picks up various contaminants, including dissolved organic compounds, heavy metals, and other pollutants.

2. Composition of Leachate:
- Leachate is a complex mixture of organic and inorganic compounds, including heavy metals, ammonia, organic acids, and various pathogens.
- Its composition depends on several factors, such as the type and age of the waste, rainfall patterns, and waste management practices.

3. Management and Treatment:
- Managing leachate is an essential aspect of landfill operations to prevent its release into the environment.
- Landfills are equipped with collection systems to capture and remove leachate from the waste mass.
- The collected leachate is then treated to remove contaminants before being discharged or reused.

In summary, leachate is an effluent from sanitary landfills. It is formed as a result of the decomposition of waste materials in the landfill and contains various contaminants. Proper management and treatment of leachate are crucial to protect groundwater and surface water quality.