The correct answer is D) upon entry into the water. The external flotation bladder on an immersion suit should be inflated upon entry into the water to provide immediate buoyancy and prevent the wearer from sinking. This is a critical safety feature that helps the wearer remain afloat and increases their chances of survival in cold water emergencies. The other answer choices are incorrect because: A) inflating the bladder after four hours is too late and could lead to drowning, B) inflating before entering the water defeats the purpose of the bladder, and C) inflating after two hours is also too late to provide effective flotation.