The correct answer is: A. Helium
Helium is the first element in the periodic table and is the second most abundant element in the universe, after hydrogen. It is a colorless, odorless, and tasteless gas that is lighter than air. Helium is non-flammable and non-toxic. It is used in a variety of applications, including balloons, cryogenics, and nuclear reactors.
Helium has only two electrons in its outer shell, which is why it is considered an inert gas. Inert gases are gases that do not react with other elements. This is because they have a full outer shell of electrons. Helium’s two electrons are paired, which means that they have opposite spins. This makes it difficult for other atoms to interact with helium’s electrons.
Argon, neon, and krypton are also inert gases, but they have eight electrons in their outer shells. This means that they are more stable than helium and are less likely to react with other elements.
Helium is the only inert gas that has only two electrons in its outer shell. This makes it unique and special.