Explanation

The question asks for the pair that is not correctly matched. Nitrogen (2p3) has three unpaired electrons, carbon is tetravalent, and chlorine has two stable isotopes (Cl-35 and Cl-37), so the first three pairs are correct. Helium, however, is a monatomic noble gas; it does not form a diatomic molecule. Hence the Helium pair in option D is the incorrectly matched one.