JEE Main · 2017 · 02 Apr 2017 · Offline · Chemical Bonding and Molecular Structure

Paramagnetism needs unpaired electrons, so build the molecular-orbital counts. $\mathrm{NO}$ with $15$ electrons has one unpaired, $\mathrm{O}{}_2$ with $16$ has two, and $\mathrm{B}{}_2$ with $10$ has two as well — all paramagnetic. Only $\mathrm{CO}$ with $14$ electrons fills every orbital in pairs, making it diamagnetic and so the species that is not paramagnetic.