The correct answer is A. Sodium.
Sodium vapor lamps are a type of high-pressure discharge lamp that emits light by ionizing sodium atoms. The light produced by sodium vapor lamps is monochromatic, meaning that it has a single wavelength. This makes sodium vapor lamps ideal for street lighting, as they can produce a bright, uniform light that is not easily scattered by the atmosphere.
Neon lamps, hydrogen lamps, and nitrogen lamps are all types of low-pressure discharge lamps. These lamps emit light by exciting electrons in a gas, causing them to emit light when they return to their ground state. However, the light produced by these lamps is not monochromatic, and it is therefore not as well-suited for street lighting as sodium vapor lamps.
In addition, sodium vapor lamps are more efficient than other types of discharge lamps. This means that they use less energy to produce the same amount of light. As a result, sodium vapor lamps are the most common type of street light used today.