The correct answer is: Neon.
Neon is a noble gas that is used in colored discharge tubes for advertisement. When an electric current is passed through a neon-filled tube, the neon atoms emit light of a specific color. The color of the light depends on the energy of the electrons in the neon atoms. The higher the energy of the electrons, the shorter the wavelength of the light and the bluer the color.
Argon, helium, and xenon are also noble gases, but they do not emit light as brightly as neon. This is because the electrons in these gases have higher energy levels, and it takes more energy to excite them. As a result, the light emitted by argon, helium, and xenon is less visible.
Neon is also used in other applications, such as light bulbs, lasers, and signs. It is a safe and non-toxic gas, and it is relatively inexpensive.