Explanation

(a)
Phenol$\overset{NaNO_{2}/H_{2}SO_{4}\ }{\rightarrow}B\overset{H_{2}O}{\rightarrow}C\ \overset{NaOH}{\rightarrow}D$
This is Liebermann’s nitroso reaction of phenol. When phenol is warmed with sodium nitrite and 1 cc. conc.$H_{2}SO_{4}$, blue colour is obtained which on adding water, becomes red. This again turns to blue on adding NaOH. Deep blue colour is due to the formation of sodium salt of indophenol.
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