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 under the requirements for immersion suits, they must be able to automatically turn an unconscious person face-up in the water within 5 seconds, without any assistance from the wearer. This is a critical safety feature to ensure the wearer's airway remains clear even if they become incapacitated. The other options are incorrect because: A) Immersion suits do not provide indefinite protection against hypothermia, B) The suits may not be resistant to all fuels and chemicals, and C) Immersion suits are not designed to be flameproof for swimming through burning oil.