The compound X−Cl (where x= aryl, alkyl, or their derivatives on reaction with sodium methoxide (NaOMe) produces X− OMe and NaCl in accordance with following chemical equation
The above reaction is a type of nucleophilic substitution reaction in which nucleophile [here OMe] replaces halogen atom of X−Cl.
Reactivity of such organic halides in nucleophilic substitution reaction depends upon polarity of C−Cl bond. Higher the polarity of C−Cl bond easier will be the nucleophilic substitution reaction, hence, faster will be the reaction here C−Cl bond in aryl halide has partially double bond character due to conjugation of lone pair of Cl with π electrons of benzene ring as shown below
Due to partial double bond character breaking of C−Cl bond aryl halide become impossible. Among the given four substrates the compound having most polar C−Cl bond will undergo faster nucleophilic substitution reaction. Greater the number of electron withdrawing group higher will be polarity of C−Cl bond.
Number of withdrawing groups
(a) has two electron withdrawing groups hence, (a) has most polar C - Cl bond and (a) will undergo fastest reaction chemical reaction occurring in the above reaction is as follows
Number of withdrawing groups 