Introduction:
Self-purification refers to the natural process by which running streams or rivers cleanse themselves from pollutants and maintain their water quality. It involves various physical, chemical, and biological mechanisms that help in the removal or degradation of contaminants. The self-purification process is important for the overall health and sustainability of aquatic ecosystems.

Explanation:
The correct answer is option A, which includes dilution, sedimentation, and oxidation as the main mechanisms of self-purification in running streams. Let's understand each of these processes in detail:

1. Dilution:
Dilution is the process of reducing the concentration of pollutants in water by mixing them with a larger volume of clean water. When pollutants enter a running stream, they get dispersed and mixed with the flowing water, resulting in a decrease in their concentration. As the contaminated water mixes with a larger volume of clean water, the overall pollutant concentration decreases, leading to the self-purification of the stream.

2. Sedimentation:
Sedimentation involves the settling of suspended particles or pollutants present in the water column. When pollutants enter a stream, they may be in the form of suspended solids. As the water flow slows down in certain areas, the suspended particles settle down due to gravity. This process is called sedimentation. It helps in the removal of larger particles, such as silt, clay, and organic matter, from the water, thereby reducing the pollutant load.

3. Oxidation:
Oxidation is a chemical process that involves the breakdown or transformation of pollutants through reactions with oxygen. Many pollutants, such as organic matter, nutrients, and some toxic substances, can be degraded or converted into less harmful forms through oxidation. In running streams, oxygen is continuously replenished through the exchange with the atmosphere and the photosynthetic activity of aquatic plants. This oxygen-rich environment supports the growth of aerobic bacteria, which play a crucial role in the degradation of organic pollutants through oxidation.

Conclusion:
In conclusion, self-purification of running streams is achieved through a combination of dilution, sedimentation, and oxidation processes. Dilution helps in reducing the pollutant concentration by mixing contaminated water with clean water. Sedimentation allows the settling of suspended particles, while oxidation facilitates the breakdown or transformation of pollutants through reactions with oxygen. Together, these mechanisms contribute to the natural cleansing and maintenance of water quality in running streams.