This landform, known as a , forms the boundary between land and the ocean.

This landform, known as a Coastline (or coast), forms the boundary between land and the ocean.

The coast is also known as the coastline and is the area where land basically connects to the sea or ocean, or we can also say that this is a line that forms the boundary between the land and the ocean or a lake. Both the terms coast and coastal are usually used to represent a geographic location. The coastline is used for agriculture, human settlement, industry,  trade, and amenity.

He/she is correct it is Coastline.