Statement A: False.
Statement B: True.
Statement C: True but problematic.
Statement D: False.
Now for the analysis:
Z is, apparently, the metal filament wire of the light bulb. The brightness of the bulb depends on two things: Surface area of the wire, and how hot it is (Stefan-Boltzmann law).
While the physical properties of the part Z are part of determining how bright the wire becomes, there is another, external factor on how hot it DOES become, and that's the current driven through the wire by the operating voltage of the bulb.
Obviously, the length, thickness, and material of the wire itself determine its resistance, but regardless that is not the only thing determining the "brightness" of the bulb. Since the current can be regulated by operating voltage, the bulb can be made dimmer or brighter regardless of the filament itself.
It
could be argued that it determines the
maximum potential brightness of the bulb because beyond certain temperature the filament disintegrates, but that's not really what statement A actually says.
Statement B is the simplest of the four and analyzing it is simplest. The only flaw is that the wire only needs high melting point if you want the bulb to work as a light source. But since that's usually the most common purpose for a light bulb as this, and I'm feeling generous, I'll take this for granted.
Statement C is troublesome because it is ambiguous. Electrical current is responsible for heating up the wire, true. And when the wire overheats, it stops working. But, it isn't always quite as simple as saying that the wire melts. It can also be vapourized, and this in fact happens slowly throughout the life cycle of a tungsten wire filament bulb, even though glass bulbs are typically filled with inert gas that slows down the vapourization of the metal. The ejected tungsten atoms accumulate on the inside of the glass bulb, which causes the typical darkening of light bulbs in use.
Statement D is unilaterally wrong, from basic physical perspective as well as linguistical.
The part Z does never "give off" energy of any kind. Much less "Light energy" which is not even a thing in the context of the school's interpretation of the statement.
First, to "give off" energy would mean that we could actually produce net energy gain by driving current through metal wire, which would act as a catalyst or something, and then the wire would magically produce energy. Obviously, this is not the case - the current requires a voltage, and the power of the radiation given by the wire is characterized by the voltage drop over the wire - or, depends on how much voltage is required to drive a current A through the wire, however you look at it it's an energy conversion process rather than the wire "giving off" energy.
Secondly, while light does have energy, what that actually is is energy of electromagnetic wave motion. Calling it "light energy" is bad use of scientific terminology, because fundamentally there's no difference between visible light, thermal radiation, microwave radiation and ultraviolet radiation.
The lamp's doing one thing - it heats up with electric current and produces a continuous spectrum of electromagnetic radiation based on its temperature. Most of the radiation is thermal radiation (longer wavelengths than visible light), part is visible light, and some parts are ultraviolet wavelengths. Fundamentally, all of these are same form of energy and it is
profoundly misleading to separate them into "light energy", "heat energy", and... what, ultraviolet energy? Microwave energy? What nonsense.
Which leads us to the interpretation of the statement's wording. The question whether the "only" refers to "only when an electric current" or "light energy only" is something that really shouldn't even be asked, so we can probably agree that whichever was the writer's intention, the execution is lacking in clarity.
To me, it would be obvious that the "only" refers to the following part of the sentence.
As shown above, though, the statement has deeper problems than that.
If we assume that it should mean "only light energy", the statement could be construed as false because there's no such thing as "light energy", or true because all electromagnetic wave motion could be understood to be "light" depending on student interpretation.
In this case, the statement should actually be "It gives off only visible light when an electric current passes through it."
On the other hand the more natural assumption of "only when" would mean that the wire produces light only when electric current passes through it.
This is strictly speaking not true. Several counter-examples can be used to show this: Physically speaking, the wire will produce light when it's hot enough to do so. It can be heated with electric current going through it, or by lasers, or it could act as an antenna and driven to oscillate by suitable frequency of radio waves. Or, the simplest case - residual heat... The lamp will continue to glow, albeit for a short time, after current has been disconnected. In fact, in the case of bulb being operated with alternating current, this is the only reason why the bulb produces continuous light despite being driven by current that essentially turns on and off 50 times a second.
The writer of this exam question should be deeply ashamed regardless of their ability with English language.
