Which of the following reactions is used to prepare salicylaldehyde...

Reimer-Tiemann reaction is used to prepare salicylaldehyde from phenol.

In this reaction, phenol is treated with chloroform ($CHCl_3$) in the presence of aqueous sodium hydroxide ($NaOH$) at $340 \text{ K}$, followed by hydrolysis, to yield salicylaldehyde (2-hydroxybenzaldehyde).

Kolbe's reaction is used to prepare salicylic acid.

Williamson synthesis is used for the preparation of ethers.

Esterification is used for the preparation of esters.

Answer: Reimer-Tiemann reaction