The correct answer is option 'A' - Sodium.

Explanation:
Yellow lamps, also known as high-pressure sodium lamps, are commonly used as street lights. These lamps produce a yellowish-orange light that is ideal for outdoor lighting applications. The gas used in these lamps is sodium.

Sodium lamps work on the principle of gas discharge. Inside the lamp, there is a small amount of sodium metal along with a mixture of inert gases, such as xenon or neon. When an electric current is passed through the lamp, it excites the sodium atoms, causing them to emit light.

The use of sodium gas in these lamps is based on its unique property of emitting yellow light when it is excited. When the electric current passes through the lamp, it ionizes the sodium atoms, causing them to release electrons. These electrons collide with the sodium atoms, causing them to transition from a high energy state to a lower energy state. In this process, the sodium atoms release energy in the form of light.

The characteristic yellow color of the light emitted by sodium lamps is due to the specific energy levels of the sodium atoms. Sodium emits light primarily in the yellow part of the spectrum, with wavelengths around 589 nanometers. This wavelength corresponds to the yellow color that is commonly associated with sodium lamps.

The use of sodium lamps as street lights has several advantages. They are energy-efficient, providing a high amount of light output for the amount of energy consumed. Sodium lamps also have a long lifespan, reducing the need for frequent replacements. Additionally, their yellow light has been found to be less disruptive to nighttime wildlife and human circadian rhythms compared to other types of street lights.

In conclusion, the gas used in yellow lamps or high-pressure sodium lamps is sodium. The excitation of sodium atoms through an electric current causes them to emit yellow light, making sodium lamps an ideal choice for street lighting applications.