The correct answer is B. The marine inspector may require any boiler to be drilled to determine its actual thickness any time its safety is in doubt. This is correct because the regulations allow the marine inspector to require drilling of a boiler if they have any concerns about its safety, regardless of the boiler's age or previous inspections. The other options are incorrect - A is too strict, C is specific to a 10-year-old water-tube boiler, and D is about non-destructive testing which is not the main focus here.