What is Schiff's reagent?

Identify the reaction in which the carbonyl group of aldehydes and ketones is reduced to a methylene group upon treatment with hydrazine followed by heating with sodium hydroxide in ethylene glycol.

Arrange the following compounds in increasing order of their reactivity towards $HCN$:
$I$. Acetaldehyde
$II$. Acetone
$III$. Di-tert-butyl ketone
$IV$. Methyl-tert-butyl ketone

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

Correct order for nucleophilic addition reaction :
$(1)$ $C_{6}H_{5}COC_{6}H_{5}$
$(2)$ $C_{6}H_{5}CHO$
$(3)$ $p-CH_{3}C_{6}H_{4}CHO$
$(4)$ $p-CH_{3}OC_{6}H_{4}CHO$

Cyclopentanone $\xrightarrow[\text{traces of } KOH]{HCN} A$ $\xrightarrow{LiAlH_4} B$ $\xrightarrow{NaNO_2, HCl} C$
$C$ is