Among the following compounds the one that is most reactive towards...

Presence of electron releasing group like $-R,-\mathrm{OH}$ etc., increases the electron density at $o / p$ position and thus, makes the benzene ring more reactive (at $o / p$ positions) towards electrophile. On the other hand, electron withdrawing group like $-\mathrm{COOH},-\mathrm{NO}_2$ etc., if present, reduces electron density and thus, reduces the activity of benzene nucleus towards electrophile. Thus, the order of the given compounds towards electrophilic nitration is
Thus, toluene is most reactive towards electrophilic nitration.