Under 46 CFR, when must the after stern tube bearing of a vessel with propelling machinery located amidship be rebushed, if the shaft diameter is 254 mm (10 inches) and the bearing is water lubricated (non-rubber)?

Why B is correct

46 CFR §61.20-23(a)(1) requires rebushing when clearance reaches 7.95 mm (0.3125 in) for shafts exceeding 229 mm (9 in) but not exceeding 305 mm (12 in) in diameter, with machinery located amidship. A 254 mm shaft falls in that range. The 6.4 mm limit applies to shafts 229 mm or less, and the 9.53 mm limit applies to shafts over 305 mm. The half-groove-depth criterion applies only to rubber bearings.