Which of the following acids was historically used by ancient Egyptians for skin conditioning?

Lactic acid is the correct choice because it has a historical context linked to ancient Egyptian practices, particularly in the use of sour milk for skin conditioning. Ancient Egyptians recognized the benefits of lactic acid, a natural alpha hydroxy acid, which is formed through the fermentation of lactose. This acid is known for its exfoliating properties and ability to improve moisture content in the skin, making it a valuable ingredient for promoting a smoother and more hydrated complexion.

In contrast, salicylic acid primarily comes from willow bark and was not noted for such historical use in skin conditioning by ancient Egyptians. Glycolic acid, while common in modern skincare, also does not have historical ties to ancient Egyptian practices. Phenol, a strong carbolic acid, is more associated with chemical peels and severe exfoliation rather than the mild, natural skin conditioning methods employed by ancient cultures. Thus, the association of lactic acid with historical skincare practices makes it the appropriate answer in this context.