Which of the following sets of reagents,used in the order shown,would successfully accomplish the conversion shown?

Which of the following aldehydes gives a red precipitate with Fehling solution?

Compound $(A)$ with molecular formula $C_6H_{12}O_3$ gives a positive iodoform test and a negative Tollens' test. Upon hydrolysis with a drop of $H_2SO_4$,it gives a product that shows a positive Tollens' test. Compound $(A)$ is:

The product of the above reaction is

Give applications of cyanohydrins with acetaldehyde with suitable reactions. $OR$ Give following conversions by using acetaldehyde as starting material.
$(a)$ Lactic acid and acrylic acid
$(b)$ Alanine
$(c)$ $1$-amino propane-$2$-ol

Complete reaction of acetaldehyde with excess formaldehyde,upon heating with conc. $NaOH$ solution,gives $P$ and $Q$. Compound $P$ does not give Tollens' test,whereas $Q$ on acidification gives positive Tollens' test. Treatment of $P$ with excess cyclohexanone in the presence of catalytic amount of $p$-toluenesulfonic acid $(PTSA)$ gives product $R$. Sum of the number of methylene groups $(-CH_2-)$ and oxygen atoms in $R$ is....