When atoms form a bond, it is usually because each atom acquires a more stable arrangement of electrons in its electron shells.
what is the best description of this arrangement of electrons?
a.
all shells have the same number of electrons.
b.
the inner shells are empty.
c.
the outer shell is completely full.
d.
all shells are half full.

C. Usually when an atom loses or gains an electron, it is because it is trying to satisfy the Octet Rule. The Octet Rule states that an atom is at its stablest when it has 8 valence electrons (two in helium's case)

If you look on the periodic table, elements on the left (Alkaline Metals) are the most reactive because they only have one valence electron (or electron in the outer shell). Elements on the right (Noble Gases) are the least reactive because they have a full outer shell of 8 valence electrons.

Later on you will find that as stability decreases as you go down the periodic table but that is a discussion for a different time.