Which of the following is a function of ultrasound technology in esthetics?

Ultrasound technology in esthetics is primarily utilized for its ability to enhance the penetration of products into the skin, stimulate tissue repair, and promote circulation, which is beneficial for various treatments. In the context of this question, its function in cellulite reduction is well-recognized. The ultrasound waves create vibrations that can help break down the lipids in fat cells, reduce the appearance of cellulite, and improve the overall texture of the skin.

The other options do not accurately represent a primary function of ultrasound technology. Facial hair removal typically involves laser or electrolysis methods rather than ultrasound. Astringent application pertains to topical solutions that may tighten the skin, rather than a treatment modality, and skin whitening is generally achieved through various topical treatments or peels, not ultrasound. Thus, the targeted action of ultrasound technology makes it particularly effective in assisting in the reduction of cellulite.