The electron withdrawing power (−I effect) of various functional groups follows the standard experimental order:
−NO2>−CN>−COOH>−F>−Cl>−Br>−I
Based on this series, the increasing order of electron withdrawing power for the given groups is:
−I<−COOH<−CN<−NO2
This corresponds to the sequence d<b<a<c.
Answer: d<b<a<c