The correct answer is: A. Atomic emission.
Mercury and sodium street lamps light up because of a process called atomic emission. This is when an atom is excited by an external source of energy, such as heat or electricity, and then emits light as it returns to its ground state. The color of the light emitted depends on the type of atom and the amount of energy it absorbs.
In the case of mercury and sodium street lamps, the atoms are excited by an electric current passing through a gas of mercury or sodium vapor. The excited atoms then emit light of a specific wavelength, which corresponds to the color of the light we see.
Atomic absorption is a different process in which an atom absorbs light of a specific wavelength. This can be used to identify the presence of an element in a sample.
Electron emission is the process in which an electron is ejected from an atom or molecule. This can be caused by a number of things, such as heat, light, or electricity.
Electron absorption is the process in which an electron is captured by an atom or molecule. This can also be caused by a number of things, such as heat, light, or electricity.