Explanation:

Self-purification of running streams refers to the process of natural purification of water in a stream due to various physical, chemical and biological processes. The correct option for the factors responsible for self-purification of running streams is B, i.e., dilution, sedimentation and oxidation. Let's understand each of these processes in detail.

Dilution:
Dilution is the process of reducing the concentration of pollutants in water by adding fresh water. When pollutants enter a stream, they get diluted as they mix with the flowing water. Dilution reduces the concentration of pollutants and makes the water less harmful to aquatic life.

Sedimentation:
Sedimentation is the process by which suspended solids settle down to the bottom of a stream. As water flows in a stream, it carries suspended particles such as soil, sand, and silt. These particles settle down due to gravity and form a layer of sediment at the bottom of the stream. The sediment layer acts as a natural filter and removes impurities from the water.

Oxidation:
Oxidation is the process of converting harmful pollutants into less harmful substances by adding oxygen. When organic matter such as sewage enters a stream, it consumes the dissolved oxygen in the water, making it difficult for aquatic life to survive. However, natural processes such as photosynthesis and aeration add oxygen to the water, which helps in the breakdown of organic matter and the removal of pollutants.

Coagulation:
Coagulation is the process of clumping together of small particles to form larger particles that can be easily removed. In a stream, coagulation occurs due to the presence of naturally occurring substances such as algae, clay, and bacteria. These substances cause small particles to stick together, forming larger particles that settle down to the bottom of the stream.

Conclusion:
In conclusion, the self-purification of running streams is a complex process that involves various physical, chemical, and biological processes. Dilution, sedimentation, and oxidation are the primary factors responsible for the natural purification of water in a stream. These processes work together to remove pollutants and make the water safe for aquatic life.