How is a molecular/covalent compound formed?

Question

$\begin{array}{1 1} \text{a.Formed when elements are combined} \\ \text{ b. Formed when non metals gain electrons} \\ \text{ c. Formed when metal atoms share electrons to complete the outer shell} \\ \text{ d. Formed when non-metal atoms share electrons to complete the outer shell} \end{array}$