Which of the following is a primary type of contour line?

The primary type of contour line is the index contour. Index contour lines are typically drawn thicker and labeled with specific elevation values. They are used to indicate the elevation of the terrain and to help users quickly understand the relief features of a map.

Contour lines in general represent elevation on a map, with each line indicating a specific elevation above sea level. Index contours help highlight changes in elevation more clearly and make it easier for map readers to interpret the landscape. They serve as reference points for measuring elevation between them and assist navigators and outdoor enthusiasts in understanding the topography of the area.

The other terms listed may refer to specific line types or concepts in mapping or navigation but do not represent primary types of contour lines in the same way that index contour lines do. For instance, supplementary contour lines are often used for more detailed surveying and aren't primary indicators, while a baseline is typically associated with the starting point for measurements rather than representing a topographical feature.