The breakdown of food into molecules small enough to enter the bloodstream is primarily accomplished by the
stomach
small intestine
large intestine
liver

The correct answer would be a small intestine.

The small intestine is the principal organ of digestion as well as absorption.

Events take place in the small intestine:

Intestinal enzymes such as maltase, lactase, sucrase et cetera digest disaccharides to their respective monosaccharides such as glucose, fructose, galactose. Dipeptides are digested to amino acids. Intestinal lipases convert di and triglycerides into fatty acids and glycerols. Lastly, nucleotide and nucleoside are converted their respective sugars and bases. The digested food is then absorbed with the help of diffusion, facilitated diffusion, and active transport. The mucosa layer of small intestine consists of finger-like projections called villi and microvilli. They increase the surface area of the small intestine and help in absorbing the digested food.  They then transport the absorbed materials to the bloodstream through capillaries and lymphatic vessels lying close to their surface.

Iguess i would say diagrams would be most difficult subject too correct