Answer: Ultraviolet Light
Ultraviolet light, whose wavelength is approximately between 100 nm and 380 nm; is a type of electromagnetic radiation that is not visible to the human eye.
This light is used for many purposes, among which is the identification of fluerescent minerals.
In this sense, fluorescence is a property that certain materials have in which they absorb energy in the form of short wavelength not visible electromagnetic radiation (the ultraviolet, for example) and then emit some of that energy in the form of longer wavelength electromagnetic radiation (in the visible spectrum). This is also called luminiscence.
Hence, the correct option is a.