Explanation

(b)
When sodium ethoxide reacts with iodoethane, diethyl ether is obtained (Williamson’s synthesis) The mechanism of this reaction is as follows
$C_{2}H_{5}ONa \rightleftharpoons C_{2}H_{5}O^{-} + Na^{+}$
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Since, the reaction involves substitution of a group by a nucleophile, it is an example of nucleophilic substitution reaction.