The correct option is option a bedrock.
Explanation- The sand present on the beaches has to stay there and stick to something to form ground. This ground is the Bedrock present at the beaches which offer a solid grounding over which the sand is held on to.
If this Bedrock would not have been present and would have some must completely or it would have been really difficult to stand as standing on it would have led to submerging of legs into the sand.