The correct answer is A) of maximum longitudinal bending moments. The keel scantlings, which refer to the thickness and structural reinforcement of the keel, are greatest amidships because this is the region that experiences the maximum longitudinal bending moments. The keel is the primary structural member that runs the length of the vessel and must be able to withstand the greatest bending stresses, which occur at the midship section where the vessel's weight and buoyancy forces are in balance. The other answer choices are incorrect because they do not directly relate to the reason for the increased keel scantlings amidships. Connections between the forebody and afterbody (B), resistance to grounding (C), and racking stresses (D) are not the primary factors determining the keel scantling requirements.