Covalent bonds share electrons between two atoms, or rather, one completes the missing electron in the other. So you'll find that determining where each atom lies in the magic table makes a difference here. For example: a) C2H6 C has a +4 and H, -1, so when two carbon atoms connect to each other, that's +6 left, so 6 hydrogen atoms will complete that. b) S has +2 and F has +1 so four Flouride atoms are not forming covalent bonds with one Sulfur atom. c) oxygen each has +2 so this is a covalent bond. d) Mg has -2 and N has +3 so three magnesium atoms (-6) and two nitrogen atoms (6) is going to form a covalent bond.
That means the answer is b.