The correct answer is B) rigidly mounted, Babbitt lined, split type. Main propulsion reduction gear bearings are typically rigidly mounted, Babbitt lined, and split type construction. This design allows for easy installation and maintenance of the bearings. The Babbitt lining provides a soft bearing surface that can conform to the shaft, while the rigid mounting ensures proper alignment of the reduction gears. The split construction enables the bearings to be removed and inspected without having to disassemble the entire reduction gear system. The other answer choices are incorrect because: A) self-aligning bushings are not commonly used for main propulsion reduction gear bearings, C) spherical-seated, tapered roller bearings are not typically found in this application, and D) self-lubricating, sealed, roller ball bearings are not the standard design for main propulsion reduction gear bearings.