The correct answer is D) upon entry into the water. The external flotation bladder of an immersion suit should be inflated upon entry into the water in order to provide immediate buoyancy and prevent the wearer from sinking. This is a critical safety feature of immersion suits, as it helps ensure the wearer remains afloat and can be easily spotted by rescue personnel. The other answer choices are incorrect because they do not provide the necessary buoyancy at the critical moment of entering the water.