Erosion forms gullies primarily through the process described in option B: sediment was washed away by water.
When heavy rainfall occurs, water flows over the surface of the land. If the water begins to flow steadily in a particular direction, it can start to remove soil and sediment from the surrounding area. This process is called erosion.
As the water continues to flow, it can carve deeper channels into the ground, creating gullies over time. The constant removal of sediment and soil by the moving water not only deepens the channels but also widens them, leading to the formation of pronounced gullies.
In summary, gullies are primarily formed as water washes away sediment, leading to the shaping of the landscape through erosion.