Which among the following is the most deactivating meta-directing group in aromatic substitution reaction?

The correct option is -NO₂.

In electrophilic aromatic substitution (EAS) reactions, substituent groups attached to the benzene ring influence both the reactivity of the ring (activation or deactivation) and the position where the new electrophile will attach (orientation).

Substituents that withdraw electron density from the benzene ring are deactivating groups because they decrease the electron density of the ring, making it a weaker nucleophile towards electrophiles. These electron-withdrawing groups also direct incoming electrophiles specifically to the meta position, because they destabilize the carbocation intermediates formed during ortho and para attacks much more than those formed during meta attack.

The strength of deactivation depends on the electron-withdrawing capability of the group through resonance (-R) and inductive (-I) effects. Let's compare the given meta-directing groups:
1. Nitro group (-NO₂): The nitrogen atom has a formal positive charge and is directly attached to the benzene ring. It strongly withdraws electron density through a powerful negative resonance effect (-R) and a strong negative inductive effect (-I).
2. Sulfonic acid group (-SO₃H): The sulfur atom is bonded to highly electronegative oxygen atoms, drawing electron density away, but the deactivating effect is weaker than that of the nitro group.
3. Cyano group (-CN): The carbon atom is triple-bonded to nitrogen, withdrawing electron density via both inductive and resonance effects, but it is less deactivating than the nitro group.
4. Carboxylic acid group (-COOH): The carbonyl carbon withdraws electron density, but its deactivating power is relatively weak compared to the nitro group.

Therefore, the order of deactivating strength among these groups is:
-NO₂ > -CN > -SO₃H > -COOH

Thus, the nitro group (-NO₂) is the most strongly deactivating meta-directing group among the given options.