The correct answer is D. River reaches.
A river reach is a segment of a river that is relatively uniform in terms of its physical characteristics, such as channel width, depth, and gradient. River reaches are typically delineated by changes in these physical characteristics, such as the confluence of two rivers or the presence of a major tributary.
Mesohabitats are smaller units of habitat that are within a river reach. They are typically defined by the presence of certain physical features, such as riffles, pools, or backwaters. Mesohabitats can be important for the distribution and abundance of aquatic organisms, as they provide a variety of habitats with different environmental conditions.
Patches are even smaller units of habitat that are within a mesohabitat. They are typically defined by the presence of certain plant communities, such as algae, macrophytes, or woody debris. Patches can be important for the distribution and abundance of aquatic organisms, as they provide a variety of habitats with different food resources and shelter.
In conclusion, river reaches are the largest spatial units in terms of scale. They are followed by mesohabitats and patches.