What does the term 'gunwale' refer to in boat anatomy?

The term 'gunwale' refers to the upper edge of the sides of a boat. It plays a critical role in providing structural support to the vessel and serves as a guard to prevent water from entering the boat. Understanding this term is essential for anyone involved with boating, as it relates not just to the design and stability of the vessel but also to safety features, as the gunwale can be a critical point when securing equipment or managing waves.

In this context, the other options address different parts of a boat, such as the rear (stern), the lower section of the hull (keel or bottom), and the base of the mast (step). However, none of these terms refer to the side margin of the boat, which is specifically what 'gunwale' denotes. This distinction is important for clarity in nautical terminology.