For each of the following compounds, indicate the aldehyde or ketone from which it would be formed

Question:

For each of the following compounds, indicate the aldehyde or ketone from which it would be formed by an aldol addition:
a. 2-ethyl-3-hydroxyhexanal
b. 4-hydroxy-4-methyl-2-pentanone
c. 2,4-dicyclohexyl-3-hydroxybutanal
d. 5-ethyl-5-hydroxy-4-methyl-3-heptanone