Identify the reagent used in the following reaction:

$CHCl_3 +aq.NaOH$

The correct answer is Option 4. $CHCl_3 +aq.NaOH$.

When phenol is treated with chloroform (\(CHCl_3\)) in the presence of \(NaOH\), it undergoes a Reimer-Tiemann reaction. In this reaction, phenol reacts with chloroform in the presence of a strong base like sodium hydroxide to form salicylaldehyde as the major product. The Reimer-Tiemann reaction can be represented as:

Mechanism:

It involves electrophilic substitution of the aromatic nucleus, the electrophile being dichloro carbene \((:CCl_2)\) (carbon has only sextet of electrons), formed by the action of strong alkali on chloroform.