According to 46 CFR §61.20-23, a vessel with propelling machinery located amidship has a tailshaft of 260 mm (approximately 10.25 in) in diameter. At what after stern tube bearing clearance must the bearing be rebushed?

Why A is correct

46 CFR §61.20-23(a)(1) sets the rebushing clearance at 7.95 mm (0.3125 in) for shafts exceeding 229 mm (9 in) but not exceeding 305 mm (12 in) in diameter with machinery amidship. A 260 mm shaft falls in this range. The 6.4 mm limit applies to shafts 229 mm or less, and the 1.6 mm reduction applies only when machinery is located aft.