Among the following species, the diamagnetic molecule is:

🧠 Diamagnetic = all electrons paired. Use MO theory for each species.

🗺️ Check unpaired electrons:

| Species | Total e⁻ | Unpaired e⁻ | Magnetic | |---------|-----------|------------|---------| | CO | 14 (isoelectronic with $\mathrm{N_2}$) | 0 | Diamagnetic ✓ | | $\mathrm{B_2}$ | 10 | 2 (in degenerate $\pi_{2p}$) | Paramagnetic | | $\mathrm{O_2}$ | 16 | 2 (in degenerate $\pi^*_{2p}$) | Paramagnetic | | NO | 15 | 1 | Paramagnetic |

⚡ CO has the same number of electrons as $\mathrm{N_2}$ (both 14e), so its MO configuration is identical: all electrons paired → BO = 3, diamagnetic.

$\boxed{\text{Answer: CO}}$