What characterizes the clean water stage in the self-purification process?

The clean water stage in the self-purification process is characterized by a significant reduction in pollution levels, resulting in water that is much cleaner and more conducive to supporting aquatic life. At this stage, while water may still contain some bacteria, these organisms are generally indicators of a healthier ecosystem rather than pollutants. The presence of bacteria can be associated with the decomposition of organic matter, which is a natural part of the self-purification process.

In this stage, the water has undergone enough biological and chemical processes to have its quality improved, making it suitable for various uses and for sustaining fish populations and other aquatic organisms. Hence, this stage represents a transition toward a more balanced ecosystem where the water is no longer considered polluted in the traditional sense.

In contrast, visible pollution suggests an unhealthy water body, low dissolved oxygen levels indicate a lack of sufficient oxygen necessary for aquatic life, and the absence of fish populations would point to a severely degraded environment rather than a clean water stage. Therefore, the option that recognizes the presence of bacteria alongside an overall improvement in water quality accurately describes the clean water stage.