Final answer:
Chemical digestion corresponds to the use of enzymes to break down complex molecules (option c), and this primarily occurs in the small intestine where most absorption also takes place (option b).
Step-by-step explanation:
The digestive process called chemical digestion matches with the description of using enzymes to break down complex molecules into simpler ones that can be absorbed. Hence, chemical digestion corresponds to the process described in option (c) "Use of enzymes to break down complex molecules". The breakdown of large food molecules like proteins, lipids, nucleic acids, and starches into subunits such as amino acids, fatty acids, glycerol, nucleotide bases, and simple sugars is accomplished through hydrolysis by digestive enzymes primarily in the small intestine. This enzymatic activity begins in the mouth with saliva and continues in the stomach, but most chemical digestion occurs in the duodenum, the first part of the small intestine.
Absorption, on the other hand, which matches description (b) "Absorption of nutrients in the small intestine", refers to the process where the small nutrient molecules resulting from the digestive process are taken up by the blood. After absorption, these nutrients travel in the bloodstream to cells throughout the body.
The breaking of large food particles into smaller ones, as mentioned in option (a), is associated with mechanical digestion, which is a physical process starting with chewing in the mouth and continuing with mixing movements in the stomach.
The movement of food through the digestive tract, indicated as option (d), is not a digestive process but rather the physical progression of food items known as peristalsis, which is not directly related to the enzymatic breakdown of food.