The correct answer is - B) the water quality of the stream.
By studying the fish populations and their well being, the watersheds, as well as the composition of the water, the scientists are trying to keep track of the situation of the quality of the water in the stream.
The fish populations, their activity, well being, are always a nice indicator about the water quality, as they live in it and are directly effected by any negative changes in the water quality.
The watersheds can also show a partial picture of the quality of the water in the streams, and that is because the water passes through them before entering the stream itself, so a clean and healthy watershed, often means good quality of the water.
The most relevant indicator of the quality of the water in the stream is still the testing of the water itself, where the water composition can be examined in detail thus to be determined its condition.