Final answer:
Glycolic acid can penetrate into the epidermis more effectively due to its small molecular size.
Step-by-step explanation:
The AHA that can penetrate into the epidermis more effectively because it has the smallest molecular size is **Glycolic acid**. Glycolic acid is the smallest AHA molecule and has the ability to easily pass through the skin's layers, reaching the epidermis effectively.
Other AHAs like Lactic acid, Citric acid, and Malic acid have larger molecular sizes compared to Glycolic acid. This means that their ability to penetrate into the epidermis is not as effective as Glycolic acid.