Silver mirror test can be used to distinguish between

The final product $A$ formed in the following multistep reaction sequence is:

In the presence of a dilute base,$C_6H_5CHO$ and $CH_3CHO$ react together to give a product. The product is

Mandelonitrile is obtained by the reaction between hydrogen cyanide and

Arrange the following compounds in the increasing order of their reactivity towards the nucleophilic addition of $HCN$:
$1. HCHO$
$2. CH_3COCH_3$
$3. PhCOCH_3$
$4. PhCOPh$

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