Final answer:
A contour line is an isoline that denotes all points at same elevation on a topographic map, with closer lines indicating steeper terrain.
Step-by-step explanation:
The question asks about a type of isoline that denotes all points at the same elevation on a map. The correct answer is A) a contour line. Contour lines on topographic maps represent lines of equal gravitational potential energy per unit mass, which is essentially the same elevation.
These lines are drawn such that as the distance between two different contour lines decreases, indicating that the steepness of the terrain increases. An area with closely spaced contour lines indicates very steep terrain, much like the sides of Devil's Tower in Wyoming. If the terrain had a constant slope, contour lines would be evenly spaced on the map.