Among the following compounds the one that is most reactive towards electrophilic nitration is:

Reactivity towards electrophilic substitution (like nitration) depends on the electron density of the benzene ring:Toluene: The $-{\text{CH}}_3$ group is an electron-donating group ($+I$ and hyperconjugation). It activates the ring.Benzene: Standard reference.Benzoic acid: The $-\text{COOH}$ group is electron-withdrawing ($-M$ and $-I$). it deactivates the ring.Nitrobenzene: The $-{\text{NO}}_2$ group is a very strong deactivator ($-M$).Thus, Toluene is the most reactive.