Answer:
The answer is B - all the available water in a stream.
Natural interest is the level of available resources in an area. We know A is not natural as it was placed there by a person. The reason it is not C is because of the total number of fish in a lake is not necessarily indicative of the natural interest of the area.
For example, the amount of fish in any single lake could be impacted by human beings bringing in additional fish to the lake, making the natural interest not an accurate estimate.
As for D, the same logic applies. The amount of oxygen in an area is not the best indicator of the natural interest of an area.