Can you say why does NO2 group show its effect only at ortho- and para- positions and not at meta- position?

Hello Student
the nitro group is an EWG (electron-withdrawing group) and hence meta-directing. When you draw the resonance structures, you can see that the ortho and para positions are those that can hold the electron density or positive charge that a substituent group provides. Since NO₂is an EWG, it will pull the electron density away from the o- and p-positions, hence electrophilic sustituents tend to attack the relatively electron rich meta-position.