The correct answer is B) absorb invisible ultraviolet light and emit visible white light. The inner phosphor coating of a fluorescent tube is designed to absorb the invisible ultraviolet (UV) light produced by the electric discharge within the tube and convert it into visible white light. This is the primary function of the phosphor coating, as it is necessary for transforming the UV radiation into the desired white light output. The other answer choices are incorrect because they do not accurately describe the purpose of the phosphor coating. Option A is incorrect because the phosphor coating does not absorb visible UV light. Option C is incorrect because the phosphor coating emits visible white light, not invisible white light. Option D is incorrect because the phosphor coating absorbs invisible UV light, not visible UV light.