Aldol condensation will not take place in

$m$-chlorobenzaldehyde on treatment with $50 \% KOH$ solution yields

Which of the following reactions does not represent the aldol condensation reaction?

$A$ compound contains an $\alpha$-hydrogen atom and forms a $\beta$-hydroxy aldehyde in the presence of a dilute alkali. When this product is heated with a dilute acid,it yields unsaturated crotonaldehyde. Identify the compound.

Identify the suitable reagent for the following conversion:

Acetaldehyde when treated with dilute $NaOH$ gives: