The correct answer is D) The Babbitt is centrifugally spun into the bearings or cast under a pressure head. This is the correct method used to securely fasten the Babbitt lining of a reduction gear bearing to its shell. The Babbitt, which is a soft alloy used as a bearing surface, is either spun into the bearing shell at high speed to form a tight fit, or it is cast into the shell under pressure to ensure a secure bond. This manufacturing process creates a strong, long-lasting connection between the Babbitt and the bearing shell. The other options are incorrect because they do not accurately describe the standard method for securing the Babbitt lining. Option A refers to a hydrodynamic oil wedge, which is not the primary means of fastening the Babbitt. Options B and C describe alternative methods that are not the industry standard.