Final answer:
The small intestine is long with folds called villi and microvilli to maximize surface area, which increases nutrient absorption efficiency.
Step-by-step explanation:
The small intestine is essential for nutrient absorption in the digestive system. The reason why the small intestine is so long, and why it contains folds is A) Increases nutrient absorption, maximizes surface area. This increased surface area is due to the presence of villi and microvilli, which are fingerlike projections and even smaller hair like structures that greatly expand the surface through which nutrients can be absorbed. The large surface area allows the efficient uptake of nutrients into the bloodstream to satisfy the body's needs.
The inner lining of the small intestine is covered with these villi, and the apical surface of each villus is full of microvilli. This arrangement makes the intestinal wall look like a brush or carpet of tiny projections, which further enhances its ability to absorb food. The folding, villi, and microvilli increase the surface area up to 600 times, making the small intestine an exceptionally efficient organ for nutrient absorption.