Paragraph: Two aliphatic aldehydes $\mathrm{P}$ and $\mathrm{Q}$ react in the presence of aqueous $\mathrm{K}_2 \mathrm{CO}_3$ to give compound R, which upon treatment with HCN provides compound $\mathrm{S}$. On acidification and heating, S gives the product shown below :
Question: $$ \text { The compound } R \text { is } $$

• Correct answer
  A

  
  
• B

  
  
• C

  
  
• D

  
  

Solution

Some inferences from above data - $a q \mathrm{~K}_2 \mathrm{CO}_3$ can facilitate aldol condensation to aliphatic aldehydes. - HCN can lead to cyanohydrin formation - $\mathrm{H}^{+}, \Delta$ can lead to hydrolysis of cyanide to acid and following esterification. So $S$ is cyanohydrin.
Carbonyl compounds (Aldol condensation) Conceptual reaction and mechanistic understanding IV

Hence, the correct answer is option: 'A'.