The correct answer is B) rigidly mounted, Babbitt lined, split type. This is the correct answer because the main propulsion reduction gear bearings in marine applications are typically of the rigidly mounted, Babbitt lined, split type construction. This design provides a secure, non-flexing mounting for the bearings to withstand the high loads and stresses encountered in marine propulsion systems. The other options are incorrect - A) self-aligning bushings are not commonly used for main propulsion reduction gear bearings, C) spherical-seated, tapered roller bearings are more often found in smaller auxiliary equipment, and D) self-lubricating, sealed roller ball bearings are not the standard for these high-load, mission-critical applications.