Which of the following reacts with alcohol in the presence of dry $HCl$ to form an acetal?

An organic compound $P$ having molecular formula $C_6H_6O_3$ gives a ferric chloride test and does not have an intramolecular hydrogen bond. The compound $P$ reacts with $3$ equivalents of $NH_2OH$ to produce oxime $Q$. Treatment of $P$ with excess methyl iodide in the presence of $KOH$ produces compound $R$ as the major product. Reaction of $R$ with excess iso-butylmagnesium bromide followed by treatment with $H_3O^{+}$ gives compound $S$ as the major product. The total number of methyl $(-CH_3)$ groups in compound $S$ is . . . .

Give the reaction of $(a)$ acetaldehyde and $(b)$ acetone with $(i)$ $NaHSO_{3}$ and $(ii)$ $HCN$.

$P$ is an optically active compound with molecular formula $C_6H_{12}O$. When $P$ is treated with $2,4-$dinitrophenylhydrazine,it gives a positive test. However,in the presence of Tollens reagent,$P$ gives a negative test. Predict the structure of $P$.

The product $(B)$ in the above reaction is:

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