Arrange the following groups in order of decreasing − − R (or) M

$1)-{\mathrm{NO}}_2$

$2)-{\mathrm{SO}}_3\mathrm{H}$

$3)-{\mathrm{CF}}_3$

$4)-\mathrm{CHO}$

Mesomeric effect, -R or –M effect groups withdraw the $\mathrm{\pi }$ electrons from the rest part of the molecule. Thus, -M effect can be compared based on electro negativity. Decreasing order of –R (or) –M effect of groups is

$-{\mathrm{NO}}_2$$>-{\mathrm{SO}}_3\mathrm{H}$$>-\mathrm{CHO}$$>-{\mathrm{CF}}_3$