Which of the following compounds can show an aldol condensation reaction?

The major product $'P'$ formed in this reaction is:

Aldehydes that do not undergo aldol condensation are:
$1$. $CH_3CH_2CHO$ (propanal)
$2$. $Cl_3CCHO$ (trichloroethanal)
$3$. $HCHO$ (methanal)
$4$. $CH_3CHO$ (ethanal)
$5$. $C_6H_5CHO$ (benzaldehyde)

In the following reaction,$C_6H_5CN$ $\xrightarrow[(ii) H_3O^+]{(i) SnCl_2/HCl} X$ $\xrightarrow{Dil. NaOH} Y$. Here,$X$ and $Y$ are:

In the given reaction sequence,$Z$ is a mixture of alcohol and acid. The reaction for the conversion of $Y$ to $Z$ is:
$CH_3CH_2OH$ $\xrightarrow[443 \ K]{\text{Conc. } H_2SO_4} X$ $\xrightarrow[(2) \ Zn/H_2O]{(1) \ O_3} Y$ $\xrightarrow[(ii) \ H^+]{(i) \ \text{Conc. } NaOH} Z$

Which of the following is used to detect aldehydes?