What line on a map joins points of equal height above sea level, helping to represent the slope of the land?

The correct answer is a contour line, which is a fundamental concept in geography and topographic mapping. Contour lines connect points that are at the same height above sea level on a map, effectively representing the three-dimensional shape of the land in a two-dimensional format. This allows for a visual understanding of the terrain's elevation changes, which helps in assessing the slope and steepness of the land.

Contour lines are particularly useful for identifying features such as hills, valleys, and plateaus. When lines are close together, it indicates a steep slope, whereas lines that are spaced further apart suggest a gentler slope. Contour lines play a crucial role in various applications, including hiking, urban planning, and environmental studies, as they provide essential information about the landscape's vertical dimension.

Other options do not accurately represent this concept; for instance, a base line typically refers to a horizontal reference line in various fields, while an elevation line is not standard terminology in cartography, and a gradient line generally describes a change over distance rather than a consistent elevation.