What is reactant $A$?

When aldehydes or ketones are treated with $C_6H_5-NH-NH_2$ (phenylhydrazine),the product formed is:

Which of the following compounds will react with ethanolic $KCN$?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: Acetal/Ketal is stable in basic medium.
Reason $R$: The high leaving tendency of alkoxide ion gives the stability to acetal/ketal in basic medium.
In the light of the above statements,choose the correct answer from the options given below:

Identify the products of the following reaction:
$HCHO + C_6H_5CHO \xrightarrow[ii. \ H_3O^+]{i. \ conc. \ NaOH} ?$

What are $X$ and $Y$ in the following reaction sequence?
$Propene$ $\xrightarrow{HBr} A$ $\xrightarrow[Dry \ ether]{Mg} B$ $\xrightarrow[2. H_3O^{+}]{1. X} Y$