Final answer:
Glycolic acid is the AHA that can most effectively penetrate the epidermis due to its small molecular size, aiding in skin exfoliation and improvement.
Step-by-step explanation:
The common AHA (Alpha Hydroxy Acid) that can penetrate into the epidermis more effectively due to its smallest molecular size is glycolic acid. Glycolic acid has the smallest molecular structure compared to lactic, citric, and malic acid, which allows it to more easily penetrate the skin.
AHAs are widely used in skin care for exfoliation, to help reduce fine lines, wrinkles, and improve the overall texture and tone of the skin.