The correct answer is A) Pivoted-shoe thrust bearing. Merchant ships typically use pivoted-shoe thrust bearings as the independent main thrust block. This type of bearing is designed to handle high axial loads generated by the ship's propulsion system. The pivoted-shoe design allows the bearing to self-align, evenly distributing the load and reducing wear. This makes pivoted-shoe thrust bearings well-suited for the demanding conditions encountered on merchant vessels. The other options are not commonly used as the main thrust block. Split-half journal bearings are more typical for radial loads, plain thrust bearings have limited load capacity, and single-piece bushings lack the self-aligning features required for this application.