The ketone shown is warmed with each amine drawn in the options in the presence of an acid catalyst. Which amine gives an enamine as the stable product?

Cyclohexanone has alpha hydrogens. A secondary amine such as piperidine cannot lose a proton from nitrogen after forming the iminium ion, so a proton is lost from the alpha carbon instead, giving an enamine. Primary amines (ethylamine, cyclohexylamine) give imines, and a tertiary amine cannot condense with the carbonyl.