The ketone shown is treated with LDA in THF at $-78^{\circ }$C and the mixture is quenched with $\mathrm{D}{}_2\mathrm{O}$. Which structure shown is the major monodeuterated product?

The ketone shown is 2-methylpentan-3-one. The alpha carbons are the CH${}_2$ of the ethyl group and the tertiary CH of the isopropyl group. LDA at $-78^{\circ }$C removes a proton from the less hindered CH${}_2$ (kinetic enolate); protonation (here deuteration) of this enolate by $\mathrm{D}{}_2\mathrm{O}$ puts one D on that carbon. The beta CH${}_3$ is not acidic and the thermodynamic site would be the tertiary CH.