Which of the following carbonyl compounds forms the most stable hydrate when $H_2O$ is added across the $C=O$ group?

The reaction $CH_3CHO \xrightarrow[Zn(Hg)/Conc. HCl]{[H]} CH_3CH_3$ is known as:

When liquid $A$ is treated with a freshly prepared ammoniacal silver nitrate solution,it gives a bright silver mirror. The liquid forms a white crystalline solid on treatment with sodium hydrogensulphite. Liquid $B$ also forms a white crystalline solid with sodium hydrogensulphite but it does not give a test with ammoniacal silver nitrate. Which of the two liquids is an aldehyde? Write the chemical equations of these reactions also.

The major product of the following reaction is

$CH_3-C \equiv CH \xrightarrow[dil. H_2SO_4]{Hg^{+2}} (P)$,$CH_3-C \equiv CH \xrightarrow[(2) H_2O_2]{(1) BH_3, THF} (Q)$. $P$ and $Q$ can be differentiated by

Predict the product of the following reaction:
$3$-hydroxycyclopentanone $\xrightarrow{Zn-Hg/HCl}$ product