Line at which the earth or sea and sky seem to meet
A). Horizon
B). Zenith
C). Fringe
D). Plinth
Horizon
One word substitute is Horizon.
Horizon: the line at which the earth's surface and the sky appear to meet.
Zenith: the time at which something is most powerful or successful.
Fringe: an ornamental border of threads left loose or formed into tassels or twists, used to edge clothing or material.
Plinth: a heavy base supporting a statue or vase.