Which of the following represents the ultimate base level for streams?

The ultimate base level for streams is best represented by sea level. This concept relates to the idea that base level is the lowest point to which a river can erode its bed. Sea level serves as the ultimate base level because it represents the point where fresh water meets ocean water, and it is the end destination for rivers, where they lose energy and sediments are deposited.

When considering local lakes or river bends, while they may influence erosion and sediment deposition at a local scale, they do not represent the ultimate limit to a river's downward erosion. Similarly, a mountain peak is not relevant as a base level; rather, it represents a high point from which streams begin to flow downward toward base level, but cannot serve as a limit to erosion in the same way that sea level does. Thus, sea level is a fundamental concept in understanding river dynamics and landscape evolution.