The correct answer is C) longitudinal. The structural member indicated by the letter L in illustration D033DG is a longitudinal. Longitudinals are continuous, fore-and-aft structural members that provide strength and support to the hull of a vessel. They run parallel to the keel and are an essential component of the ship's framing system. The other answer choices are incorrect because: A) a bilge keel is a protective keel-like structure attached to the bottom of the hull, B) a side keel is a keel-like structure running along the side of the hull, and D) a web frame is a transverse structural member that connects the longitudinals and provides additional strength to the hull.