D

Step 1: Understand the function of an electron gun. An electron gun is a device that emits a beam of electrons and can direct these electrons towards a target.
Step 2: In many applications, such as cathode ray tubes and certain types of fluorescent lighting, the emitted electrons strike specific substances that can produce visible light through the process of fluorescence.
Step 3: By definition, fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. The electron gun operates by emitting electrons which excite these substances, leading to the emission of light.
Step 4: Options A (Thermionic valve), B (Photoelectric cell), and C (Cathode ray oscilloscope) do not primarily focus on using electron beams specifically for fluorescence production.
Conclusion: Therefore, the correct answer is D (Electron gun).