The correct answer is C. According to 46 CFR Part 61, boiler mountings attached directly to the boiler plating by screwed studs and nuts shall be removed and examined every 10 years. This is the correct answer because the regulation in 46 CFR Part 61 specifically states the requirement for boiler mountings attached by screwed studs and nuts to be removed and examined every 10 years. The other answer choices do not accurately reflect the regulatory requirement. The other answer choices are incorrect because they either specify a different time frame (8 years or 4 years) or a different method of inspection (opening and removing all mountings) that is not consistent with the regulation.