The correct answer is B. Barium.
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth most abundant element in Earth’s crust, and is a relatively soft, silvery-white alkaline earth metal. Barium is highly reactive and is used in a variety of applications, including fireworks, ceramics, and glassmaking.
When barium is heated, it emits a green flame. This is due to the fact that barium has a high energy level for its electrons. When these electrons are excited, they emit light in the green part of the spectrum.
The other options are incorrect because they do not produce a green flame when heated. Sodium produces a yellow flame, potassium produces a violet flame, and mercury produces a blue flame.
In fireworks, barium is often used in combination with other metals to create different colors. For example, barium and strontium are often used together to create a red flame.