**Electrophilic Nitration**

Electrophilic nitration is a chemical reaction in which a nitro group (-NO2) is introduced into an organic compound. This reaction is commonly performed on aromatic compounds, as they can stabilize the negative charge on the nitro group through resonance.

**Reactivity of Aromatic Compounds**

Aromatic compounds are known for their high reactivity towards electrophilic substitution reactions. In these reactions, an electrophile attacks the aromatic ring, replacing one of the hydrogen atoms.

The reactivity of aromatic compounds towards electrophilic substitution depends on the electron density of the ring. Compounds with greater electron density are more reactive towards electrophilic substitution.

**Comparing the Reactivity of Compounds**

Now, let's compare the reactivity of the given compounds:

a) Toluene: Toluene is a derivative of benzene, where a methyl group (-CH3) is attached to the benzene ring. This methyl group increases the electron density of the ring through inductive effect, making toluene more reactive than benzene.

b) Benzoic acid: Benzoic acid is a derivative of benzene with a carboxylic acid group (-COOH) attached to the ring. The carboxylic acid group withdraws electron density from the ring through resonance, reducing the reactivity of benzoic acid compared to benzene.

c) Nitrobenzene: Nitrobenzene contains a nitro group (-NO2) attached to the benzene ring. The nitro group is a strong electron-withdrawing group, which significantly reduces the electron density of the ring. As a result, nitrobenzene is less reactive towards electrophilic substitution compared to benzene.

d) Benzene: Benzene is the simplest aromatic compound and is less reactive than toluene due to the absence of any electron-donating or electron-withdrawing groups.

**Conclusion**

Based on the comparison of reactivities, toluene is the most reactive compound for electrophilic nitration. The presence of the methyl group in toluene increases the electron density of the ring, making it more susceptible to electrophilic substitution reactions.