Final answer:
Stream terraces not made of bedrock are often composed of the same materials as a modern floodplain in a river system, which includes sediments like gravel, sand, and silt.
Step-by-step explanation:
Stream terraces not composed of bedrock are likely to be made of unconsolidated material, typically sediments such as gravel, sand, and silt, which are similarly found on the modern floodplain of a river system. This area is formed as a river overflows its banks during flood events, depositing sediment across the adjacent land. These deposits can later be cut by the river, leaving behind elevated terraces as it carves a new channel at a lower level.
Floodplains are crucial for rivers as they provide areas for excess water to spread out during floods. Like stream terraces, floodplains contain sediments that have been transported by the river's flow. Hence, the correct answer to the question is c. floodplain.