The correct answer is C. In the periodic table nitrogen has 5 valence electrons and needs 3 more electrons to obtain an octet. So it would form three covalent bonds with another nitrogen and that would be a triple bond.
A is incorrect because oxygen is in group 6 and only needs 2 electrons to achieve an octet. So it would form two covalent bonds with the other oxygen to give a double bond.
B is incorrect because Cl is in group 7 and only needs 1 electron to achieve an octet. So it forms a single bond with the other Cl atom.
D is incorrect because helium is in Group 8, a noble gas, which means its valence shell is completely filled, hence no bonding can occur.
