Here, CO is a stronger ligand than Cl⁻ because:
A. CO is a neutral molecule
B. CO has pi bonds
C. CO is poisonous
D. CO is more reactive

The strength of a ligand is often determined by its ability to donate electron density to a metal ion in a coordination complex. Here's how each option relates to the strength of CO as a ligand:

A. CO is a neutral molecule: This is true, but the neutrality of the molecule doesn't inherently make it a stronger ligand. Many ligands can be neutral.

B. CO has π-bonds: Yes, this is a key factor. CO has a lone pair on carbon that can be donated to the metal ion, and the π-bonds provide additional back-donation from the metal to the ligand, strengthening the bond.

C. CO is poisonous: While true, the toxicity of CO isn't directly related to its strength as a ligand. Poisonous nature doesn't determine its ability to form coordination bonds.

D. CO is more reactive: This is somewhat true, but reactivity alone doesn't necessarily correlate with being a stronger ligand. CO's ability to accept electron density from a metal center (through its lone pair) and the back-donation from the metal are more relevant.

So, option B, "CO has π-bonds," is the most directly related to the strength of CO as a ligand.