Its is b because that is how the galaxy works.
Atomic mass? I'm not shre
Answer:
Alkali and alkaline-earth cations are energetically stable with an empty valence shell. Atoms of alkali and alkaline-earth elements achieve octets after losing one (alkali) or two (alkaline-earth) electrons such that they minimize the potential energy of the system.
Guess and check, test, trial and error, completion.
