Aldol condensations involve the formation of an enolate, followed by an attack on a carbonyl carbon. In this case, there is only a single α-carbon that has an acidic hydrogen. This means that only one of the two reactants can form an enolate, as shown below. The enolate will attack the aldehyde and form a β-hydroxyketone, shown in the center below. This molecule will undergo a dehydration to form the more thermodynamically stable trans-α, β-unsaturated ketone [B].