The correct answer is C) The suit must, without assistance, turn an unconscious person's mouth clear of the water within 5 seconds. This is the correct answer because the Code of Federal Regulations (CFR) for the U.S. Coast Guard requires that immersion suits be capable of turning an unconscious person from a face-down position to a position where the mouth is clear of the water in not more than 5 seconds, without the assistance of the wearer. The other answer choices are incorrect because: A) Immersion suits are not necessarily oil-resistant and may need special treatment after exposure to certain chemicals. B) Immersion suits provide limited thermal protection and are not designed for indefinite exposure. D) Immersion suits are not flameproof and do not provide protection against swimming through burning oil.