The correct answer is (C) A shallow place; a section of a river, lake, sea, etc., where the water is very shallow. Also, a submerged sand bar, particularly one that is exposed at low tide. In addition, hidden dangers or impediments.

Shoal comes from Middle English (Scottish) shald, Old English sceald (shallow).

Examples of use:

• The chart also showed the navigator the locations of all shoals.
• If not careful, he might ground his boat on the shoal or damage the keel.