Among the following,the marked proton of which compound shows the lowest $pK_{a}$ value?

Compounds containing two $-OH$ groups on the same carbon atom are generally unstable,but which of the following is stable?

The reactant is $3$-methylpentanal. The reaction with $NaCN$ followed by $H_2SO_4$ (cyanohydrin formation) is shown below:
$CH_3CH_2CH(CH_3)CH_2CHO \xrightarrow{NaCN/H^+} CH_3CH_2CH(CH_3)CH_2CH(OH)CN$
Products of the reaction are:

Match the following:

$A$. Lucas reagent	$I$. $SnCl_2 + HCl$
$B$. Clemmensen reagent	$II$. $[Ag(NH_3)_2]^+$
$C$. Tollen's reagent	$III$. Anhydrous $ZnCl_2 + conc. HCl$
$D$. Stephen reagent	$IV$. $Zn-Hg/HCl$

Toluene (excess) undergoes the following sequence of reactions:
$(i)$ $CrO_2Cl_2, CS_2$
(ii) $H_3O^+$
(iii) $NaHSO_3$
After filtration,a residue $(A)$ is obtained. This residue $(A)$ reacts with $dil. \ HCl$ to form compound $(B)$.
Identify the structures of residue $(A)$ and compound $(B)$ respectively.

Which of the following reagents would carry out the following transformation? ($D = $ $^2H$)