Which one of the following compounds, on reaction with hydrogen...

Reaction of a carbonyl compound with $\text{HCN}$ yields a cyanohydrin.

For the cyanohydrin to be optically active, the carbon atom formed from the carbonyl carbon must be chiral, meaning it must be bonded to four different groups.

For $\text{CH}_3\text{-COCH}_3$, the cyanohydrin is $\text{CH}_3\text{-C(OH)(CN)-CH}_3$, which has two identical methyl groups and is achiral.

For $\text{H-CHO}$, the cyanohydrin is $\text{H-C(OH)(CN)-H}$, which has two identical hydrogen atoms and is achiral.

For $\text{CH}_3\text{-CHO}$, the cyanohydrin is $\text{CH}_3\text{-CH(OH)(CN)}$, which has four different groups ($\text{CH}_3$, $\text{H}$, $\text{OH}$, $\text{CN}$) attached to the central carbon. It is chiral and optically active.

For $\text{C}_6\text{H}_5\text{-CO-C}_6\text{H}_5$, the cyanohydrin is $\text{C}_6\text{H}_5\text{-C(OH)(CN)-C}_6\text{H}_5$, which has two identical phenyl groups and is achiral.

Answer: $\text{CH}_3\text{-CHO}$