The correct answer is C) serve as a crack arrestor and prevent hull girder failure. The riveting of the lower seam indicated by the letter B helps to serve as a crack arrestor. This means that if a crack were to develop in the hull plating, the riveted seam would help to slow or stop the propagation of the crack. This helps to maintain the overall structural integrity of the hull girder and prevent a catastrophic failure. The other options are incorrect because they do not address the primary purpose of the riveted seam, which is to act as a crack arrestor. Reducing construction costs (A) or providing flexibility (B) are not the main reasons for this riveting. While increasing strength in a highly stressed area (D) may be a secondary benefit, the primary purpose is to arrest crack propagation.