During bonding, why do main group elements tend to lose or gain electrons as per the octet rule?

Question

$\begin{array}{1 1} \text{ (a) To completely fill the outer electron shell with four electrons} \\ \text{(b)To completely fill the outer electron shell with eight electrons}\\ \text{ (c) To completely fill the outer electron shell with ten electrons} \\ \text{(d) To completely fill the outer electron shell with six electrons} \end{array}$