Why Do Atoms Gain and Lose Electrons to Have Eight

Why do atoms gain and lose electrons to have eight electrons in the valence shell?

A) Atoms/ions are stable when the n=2 principal level is full.
B) Atoms/ions are stable when the s and p sublevels of the valence shell are full.
C) Atoms/ions are stable when the d sublevel of the valence shell is full.
D) Atoms/ions are stable when the n=4 principal level is full.
E) Atoms/ions are stable when the s, p, and d sublevels of the 2^nd level are full.