Part of the connection between the black sea and the Mediterranean Sea?

The answer might be c.

The highest point of a hill (or mountain) is a single point.
a single point can only be precisely marked with a dot. 
putting on a contour line is impossible  on a single point because the contour line is created after multiple points that have the same hight are marked, and after that, all of those points are connected with a line that creates a closed area. so the highest point, as said above, is a single point, there are no other points with the same height, so the only possible way for it to be marked is with a dot, and there is no possibility for a contour line.

Each meridian forms half of a great circle, assuming a spherical earth. meridians intersect lines of latitude (parallels) at right angles. the spherical earth is divided into  360 degrees   east-west. lines of longitude range from 0° to  180° to the east and from 0° to -180 ° to the west of the prime meridian.