Which of the following substituent(s) has −M (Mesomeric) effect?

If an atom or group which can donate pair of electrons to other atom through conjugation (pi bond electrons) is known as +M (mesomeric) effect.

Ex:$-{\mathrm{NH}}_2$

If an atom or group which can withdrawn electrons towards itself from the compound, then it is called −M effect. Example is given below

$-\mathrm{CN}$, $-\mathrm{CHO}$ & $-{\mathrm{NO}}_2$