Respuesta :
a) a coastline represents the boundary between land and the ocean or a lake.
First of all, a coast is where land ends and the sea/lake starts.
Therefore, the coastline, is the "border", or as you said it, the boundary between the continent and a sea or in some cases a lake.
Answer:
[tex]\boxed{\bf~A - Coastline}[/tex]
Hope it helped,
BioTeacher101