Which of the following aldehydes is most reactive towards nucleophilic addition reactions?

The above conversion can be achieved by:

The given compound can be converted into the product by which of the following reagents?

Which of the following tests is $NOT$ shown by acetaldehyde?

What type of reaction is the formation of aldol from aldehyde?

$x$ is the product obtained from propanenitrile and stannous chloride in the presence of hydrochloric acid followed by hydrolysis. $y$ is the product obtained from but$-2-$ene by ozonolysis followed by hydrolysis. From the following, which product is not obtained when one mole of $x$ and one mole of $y$ react with each other in the presence of alkali followed by heating?