Line at which the earth or sea and sky seem to meet

Answer: Option A

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.