Which one among the following pairs is not correctly matched?

Helium is a MONATOMIC noble gas, not diatomic — it exists as single atoms (He) in its gaseous state, so option (d) is incorrectly matched. Nitrogen has the electronic configuration 1s² 2s² 2p³ with three unpaired electrons in the 2p subshell (correct). Carbon is tetravalent because it has four valence electrons and forms four covalent bonds (correct). Chlorine has two stable isotopes Cl-35 and Cl-37 (correct).