Which one of the following gases produces a red light when electricity flows through it ?

When electricity flows through a gas at low pressure in a discharge tube, the gas atoms become energized and emit light of a characteristic color [2]. This phenomenon occurs because the electric current ionizes the gas, creating a conductive plasma [1]. Among the noble gases, neon is specifically known for producing a bright red-orange glow when subjected to an electric discharge [3]. While other gases produce different colors—such as argon emitting blue light and helium producing a pinkish-white or yellow-gold hue—neon's distinctive red emission is widely utilized in automotive stop lights and traditional neon signs [3]. The specific color is determined by the atomic emission spectrum of the gas, which consists of particular wavelengths of light emitted as electrons return to lower energy states.

Sources

1. [2] https://en.wikipedia.org/wiki/Neon_lighting
2. [1] https://www.sciencedirect.com/topics/earth-and-planetary-sciences/gas-discharges
3. [3] https://en.wikipedia.org/wiki/Glow_discharge