Which of the following terms is associated with the lowest section of a river?

The term associated with the lowest section of a river is the "lower course." This part of the river is characterized by its wide, flat valleys, slower flow, and increased sediment deposition as the river approaches its mouth, where it may meet an ocean, sea, or lake. In this section, you will often find features such as meanders and deltas being formed due to the river's slower movement and the accumulation of sediments.

The source refers to the starting point of a river, typically located in the hills or mountains. The upper course describes the initial stages of the river's journey, where it is usually steeper, narrower, and faster-flowing, often cutting through the landscape. A tributary is any smaller river or stream that flows into a larger river, and while they contribute to the flow of a river, they are not a section of a river in itself. Thus, the term "lower course" specifically identifies the final stretch of the river system, making it the correct answer in this context.