The correct answer is B) keel. The keel is the main structural member that runs longitudinally along the bottom of the vessel, forming the backbone of the hull. In illustration D033DG, the letter "I" is clearly pointing to the keel, which is the primary structural component indicated. The other answer choices are incorrect because: A) the garboard strake is the planking that connects the keel to the hull, C) a girder is a transverse structural member, and D) a center pillar is a vertical structural member, none of which are the longitudinal keel shown in the illustration.