Ranking these four rings by how strongly their substituent pushes or pulls electron density gives a quick answer. An alkyl group like the methyl on toluene is a genuine activator, feeding electron density into the ring by hyperconjugation and speeding up any electrophilic substitution including nitration. Halogens on fluorobenzene and chlorobenzene are net deactivators despite being ortho/para directors, their strong inductive electron withdrawal outweighs their weaker resonance donation, so both react slower than benzene. The nitro group on nitrobenzene is the strongest deactivator of the four, pulling electron density away from the ring through both induction and resonance and making the ring very resistant to further electrophilic attack. Toluene is therefore the only substrate among the four that is actually activated relative to benzene, so it nitrates the fastest.
Therefore, the correct answer is Toluene.