Violet. It has a higher frequency than red light, and as such, is more likely to exceed the work function of the metal. Look up the photoelectric effect.
The photoelectric depends on the work function of the metal. If the energy of a photon (hf) is less than the work function energy then no electrons are emitted. If the frequency is above the threshold then electrons are emitted. Blue light has greater photon energy than red light and is therefore more likely to cause electron emission from more metals than red light but it depends on the workfunction.
violet because it has a higher frequency and more energy.
