Final answer:
The stage at which a cosmetologist should produce a functional, predictable, and pleasing result is during blow-drying, as it finalizes the client's hairstyle.
Step-by-step explanation:
The service essential during which you should produce a functional, predictable, and pleasing result is D) Blow-drying. This is because the blow-drying process is when the hairstyle is finished and set into place. The client expects to leave with a hairstyle that meets their expectations, which includes it being functional (suited to their lifestyle), predictable (similar to what was discussed during the consultation), and aesthetically pleasing. This result is a testament to the professional skill and dexterity of the cosmetologist or hairstylist.