Explanation

(a)
Diethyl sulphate in the presence of NaOH acts as alkylating agent, it causes alkylation of phenol to give ethyl phenyl ether which is also called phenetole.
$C_{6}H_{5}OH + NaOH \rightarrow C_{6}H_{5}O^{-}Na^{+} + H_{2}O$
$C_{6}H_{5}O^{-}Na^{+} + \left( C_{2}H_{5} \right)_{2}SO_{4} \rightarrow C_{6}H_{5}OC_{2}H_{5}ph + C_{2}H_{5}NaSO_{4}$
diethyl sulphate phenetole