Final answer:
The middle course of a river is U-shaped mainly because of erosion and deposition processes where slower flow rates lead to wider valleys formed through lateral erosion and sedimentation.
Step-by-step explanation:
The middle course of a river is often U-shaped due to the processes of erosion and deposition. As the river flows away from its source, it widens due to the increased volume of water from tributaries, and the flow slows down. This slow flow encourages sedimentation because the gravity is not strong enough to keep the sediments suspended, leading them to settle at the riverbed. At the same time, the slower water flow allows the river to meander, creating a broad U-shaped valley primarily through the process of lateral erosion which widens the river channel.
The correct answer to why the middle course of a river is U-shaped is (a) Due to erosion and deposition processes. Other options such as volcanic activity, tectonic plate movement, or gravitational forces don't typically shape the U-form of the middle river course directly, although tectonics can play a role in the overall landscape in which a river exists.