The ketone shown is treated with LDA in THF at $-78^{\circ }$C and then with acetaldehyde at low temperature; the mixture is quenched with aqueous $\mathrm{NH}{}_4\mathrm{Cl}$ at low temperature. Which structure shown is the major aldol product?

The ketone shown is cyclopentyl methyl ketone. LDA at $-78^{\circ }$C gives the kinetic enolate at the CH${}_3$ group, which adds to acetaldehyde to give the beta-hydroxy ketone $ceCH3CH(OH)CH2C(=O)$-cyclopentyl (3-hydroxy-1-cyclopentylbutan-1-one). The tertiary-site aldol would need the thermodynamic enolate, the enone requires heating or acid or base for dehydration, and a diol would need a reducing agent.