Traction, saltation, suspension and solution are the four types of fluvial __________?.

The word that completes the sentence is “transport.”

Explanation: In fluvial (river) environments, “transport” refers to the various methods by which sediment is moved through the river system. Traction involves larger particles rolling along the riverbed, saltation involves smaller particles bouncing along the riverbed, suspension involves fine particles that are carried within the water column, and solution describes the process where minerals dissolve in the water. Each of these processes plays a crucial role in shaping river landscapes.