A fluorescent dye is used for aiding inspection of parts. It helps reveal faults. The liquid penetrant is left on the surface for a sufficient time to allow the liquid to seep into any surface openings or defects. It is important to use this to detect defects that may compromise the integrity or quality of the part being inspected.
Since the fluorescent dye is applied to the whole surface any of the options given in this question may result in seeing the fluorescent result under a black light. This process is called Fluorescent Penetrant Inspection (FPI). This inspection process is low cost and robust and thereofre used widely in various scientific situations. The dye is harmless for the part being tested.