Answer:
C. Arsenic
Explanation:
Each period is terminated by a noble gas with a closed valence shell with electronic configuration ns²np⁶. Since noble gases have completely filled orbitals in the valence shell and are very stable, it becomes very difficult to alter their stable arrangement by the addition or removal of electrons. They they exhibit very low chemical reactivity
Noble gas elements are: Neon, Argon, Krypton, Xenon, Radon, Helium (2 electrons in outer shell, stable).