White light is produced in a tube by heating the filament (option B). Inside a light bulb or tube, a filament made of tungsten is heated to a high temperature by passing an electric current through it. As the filament heats up, it emits light across the entire visible spectrum, ranging from red to violet. The combination of all these wavelengths creates white light, which appears to our eyes as a mixture of colors. This process is similar to how an incandescent light bulb or certain types of lamps operate, where the thermal radiation from the heated filament produces a broad spectrum of wavelengths. Unlike options A, C, or D, which involve different processes unrelated to the generation of white light, heating the filament is a well-established method for producing visible light that mimics natural sunlight and is commonly used in various lighting applications.