Decreasing order of – IE power of given groups is

$1) \mathrm{CN}$

$2) {\mathrm{NO}}_2$

$3) {\mathrm{NH}}_3$

$4) \mathrm{F}$

-I effect: An atom or group of atoms which has greater tendency to attract $\mathrm{\alpha }$ electrons when compared with hydrogen is called electron withdrawing group. It is said to be exhibiting negative inductive effect(-I).

Ex: Halogens, like F, Cl ; $-\mathrm{OH}, -{\mathrm{NH}}_3, -\mathrm{CN}, -{\mathrm{NO}}_2, -\mathrm{CHO}$

Therefore, decreasing order of – I.E power of given groups is

${\mathrm{NO}}_2>\mathrm{CN}>\mathrm{F}>{\mathrm{NH}}_3$