Final answer:
The brush border with its microvilli in the small intestine is primarily involved in active transport, a crucial process for nutrient absorption that requires energy to move substances against their concentration gradient.
Step-by-step explanation:
The brush border of the epithelial cells in the small intestine is crucial for nutrient absorption. The multitude of extensions known as microvilli significantly increase the surface area for absorption, facilitating several transport mechanisms. Among these, active transport is particularly vital for the uptake of nutrients such as glucose and galactose, which requires energy in the form of ATP to move these substances against their concentration gradient.
Additionally, processes like facilitated diffusion, through carrier proteins, and passive mechanisms, such as diffusion for water-soluble nutrients, are also crucial for effective absorption in the small intestine. The correct answer to the question, highlighting the physiological process that the brush border supports, is active transport. The villi and microvilli, with their extensive surface area, are adapted for this efficient transfer of nutrients, which is a key component of the small intestine's role in digestion and nutrient uptake.