The correct answer is D) Pivoted-shoe thrust bearing. Pivoted-shoe thrust bearings are the most commonly used type of independent main thrust block on merchant ships. This type of bearing consists of a series of pivoted shoes that evenly distribute the thrust load across a large surface area, providing a stable and effective means of supporting the thrust of the ship's propeller shaft. This design is well-suited for the high loads and continuous operation encountered in merchant vessel applications. The other options are not as commonly used for this purpose. Split-half journal bearings are more commonly used for radial shaft support, while plain thrust bearings and single-piece bushings are generally less suitable for the high loads and stresses associated with a vessel's main thrust block.