What landform found in the upper course of a river comprises harder rock that a river flows around?

Interlocking spurs are indeed the correct choice as they occur in the upper course of a river, characterized by alternating ridges of hard rock that the river winds around. As a river begins to erode its channel, it often encounters areas of differing rock hardness. The harder rock is more resilient to erosion, causing the river to take a path around it. This leads to the formation of interlocking spurs, where the river meanders in and out around these protruding areas of hard rock, creating a pattern that resembles interlocking fingers.

Other options refer to different landforms or features: waterfalls represent a point where water cascades over a step or cliff, typically due to a transition from harder to softer rock. Floodplains are flat areas adjacent to rivers that are subject to flooding, formed by the deposition of sediment during high water events, and deltas are landforms that develop at the river's mouth where it meets a body of water, formed by sediment deposition. These features are not generated by the same processes occurring in the upper course of a river as seen with interlocking spurs.