The correct answer is D) The aftermost line shaft bearing. The aftermost line shaft bearing on merchant ships is the most likely to have both upper and lower half bearing shells. This is because the aftermost bearing supports the greatest load from the weight and forces acting on the propeller and propeller shaft. The upper and lower half bearing shells allow for better load distribution and alignment of the propeller shaft at this critical location. The other options are incorrect because: A) The intermediate bearings may have upper and lower shells, but the aftermost bearing typically carries the highest load. B) The forward most bearing typically has a simpler design with only a lower shell. C) All line shaft bearings do not necessarily have only a lower shell - the aftermost bearing is the most likely to have both upper and lower halves.