The correct answer is B. Strontium.
The green color seen in firework displays is due to the emission of light by strontium salts when they are heated. Strontium chloride is the most common strontium salt used in fireworks, but other strontium salts, such as strontium nitrate and strontium carbonate, can also be used.
When strontium salts are heated, they emit light in the green part of the visible spectrum. This is because strontium has a very high atomic number, which means that it has a lot of electrons. When these electrons are excited, they emit light with a wavelength that corresponds to the energy difference between their excited and ground states. The energy difference between the excited and ground states of strontium electrons is such that the light they emit is in the green part of the visible spectrum.
The green color of strontium fireworks is very popular, and it is often used to create a “starburst” effect. Strontium fireworks are also used to create the “green flash” that is sometimes seen at sunset.
The other options are incorrect because they do not emit green light when they are heated.
Sodium chloride, calcium chloride, and barium chloride all emit yellow, orange, and red light, respectively.