Only an aldehyde having..... can undergo the aldol condensation.

The major product of the following reaction is :
$CH_3-CH_2-CHO \xrightarrow[\text{reflux}]{\text{excess } HCHO / \text{alkali}} ?$

An organic compound with the molecular formula $C_9H_{10}O$ forms a $2,4-DNP$ derivative,reduces Tollen's reagent and undergoes Cannizzaro reaction; on vigorous oxidation it gives $1,2-$benzenedicarboxylic acid. The structure of the organic compound is:

What is reactant $A$?

Match the reactions given in column-$I$ with the suitable reagents given in column-$II$.

Column-$I$ (Reactions)	Column-$II$ (Reagents)
$(i)$. Benzophenone $\rightarrow$ Diphenylmethane	$(a)$. $LiAlH_4$
$(ii)$. Benzaldehyde $\rightarrow$ $1-$Phenylethanol	$(b)$. $DIBAL-H$
$(iii)$. Cyclohexanone $\rightarrow$ Cyclohexanol	$(c)$. $Zn(Hg) / \text{Conc. } HCl$
$(iv)$. Phenyl benzoate $\rightarrow$ Benzaldehyde	$(d)$. $CH_3MgBr, H_2O$

An aldehyde or ketone containing at least one $\alpha-H$ atom reacts with dilute $NaOH$ to form a $\beta-$hydroxy aldehyde or $\beta-$hydroxy ketone. This reaction is known as $—$