Final answer:
The inner surface of the small intestine is covered with intestinal projections called villi. Therefore, the correct option is D.
Step-by-step explanation:
These villi increase the surface area of the small intestine, facilitating the absorption of nutrients from the digested food into the bloodstream. The presence of villi is essential for efficient nutrient absorption, as it maximizes the contact between the absorptive cells of the small intestine and the nutrients in the chyme (partially digested food).
The other options are not accurate descriptions of the inner surface of the small intestine: a) is incorrect because the small intestine can absorb multiple nutrients simultaneously, b) is incorrect as hydrochloric acid is primarily secreted in the stomach, c) is incorrect as the inner surface is not smooth but has numerous folds and projections, and e) is incorrect as hormones are primarily secreted by glands outside the intestine.
Therefore, the correct option is D.