The correct answer is D. The suit must, without assistance, turn an unconscious person's mouth clear of the water within 5 seconds. This is correct because the regulations for immersion suits require that the suit be capable of turning an unconscious person face-up in the water within 5 seconds without assistance. This is a critical safety feature to ensure the wearer's airway remains clear in the event of incapacitation. The other options are incorrect because: A) the suit's exterior color is not restricted, B) the suit should not be stored in excessively hot environments, and C) the suit is not designed to protect against burning oil, which would require a different type of specialized suit.