Reaction of phenol with chloroform in the presence of dilute sodium hydroxide finally introduces which one of the following functional groups?

The reaction described is the Reimer-Tiemann reaction.

1. Reagents: Phenol is treated with chloroform ($CHCl_3$) in the presence of aqueous sodium hydroxide ($NaOH$).
2. Process: An electrophilic substitution occurs on the aromatic ring. The reaction involves a dichlorocarbene ($:CCl_2$) intermediate. The initial product is a benzal chloride derivative which is hydrolysed in the alkaline medium to form an aldehyde group.
3. Result: The final acidification yields Salicylaldehyde (2-hydroxybenzaldehyde). Thus, an aldehyde group ($-CHO$) is introduced, predominantly at the ortho position to the phenolic -OH group.

Note: If carbon tetrachloride ($CCl_4$) were used instead of chloroform, the reaction would introduce a carboxyl group ($-COOH$) yielding salicylic acid.