Final answer:
Sediment in a major river channel is most commonly moved by river flow. As the river current slows, sediment is transported and deposited, leading to the formation of features such as asymmetrical ripple marks and cross-bedding. Hence, the correct answer is option A.
Step-by-step explanation:
The process that most commonly moves sediment in a major river channel is river flow. This can be observed as rivers start at a narrow channel with faster currents and move towards a broader channel with slower currents, causing sediments to move and eventually settle. The movement of water in one direction leads to the creation of asymmetrical ripple marks, where sediment moves up the shallow side and is deposited on the steep side, aligning with the flow of the river.
Over time, this process also forms sedimentary structures known as cross-bedding, which indicate the direction of water movement. Additionally, rivers contribute to erosion and the shaping of the Earth's surface by carrying eroded materials and depositing them in different areas, such as in wide flood plains or as they approach larger bodies of water, like oceans, forming deltas or estuarine areas.