Sodium has one valence electron, and chlorine has seven valence electrons, making both atoms unstable. The two atoms aren't sharing two valence electrons, making option D wrong as well. Chlorine wouldn't donate 1 electron, as it is in need of 1 electron itself, making option C incorrect. Ultimately, option B is correct, as Sodium needs to get rid of one electron to be stable, and Chlorine needs one electron to reach a stable number of valence electrons.
