The gently sloping margins of a continent submerged beneath the sea are called continental shelves.

Here's a breakdown:

* Continental margins are the zones where continents transition into ocean basins.

* Continental shelves are the submerged, relatively shallow, gently sloping portions of the continental margins. They extend outwards from the shoreline to a point where the slope increases dramatically, marking the beginning of the continental slope.

Think of it like this: Imagine a cake sitting on a plate. The cake represents the continent, the plate represents the ocean basin, and the area where the cake meets the plate is the continental margin. The gently sloping area of the cake touching the plate is the continental shelf.