Which process involves breaking down food into smaller units?

The process that involves breaking down food into smaller units is digestion. During digestion, larger food molecules are broken down into smaller components, such as carbohydrates, proteins, and fats, which can be more easily absorbed by the body. This process occurs through mechanical means, like chewing, and chemical means, such as the action of digestive enzymes that break down the food into its constituent nutrients.

Absorption, while closely related, specifically refers to the process where these smaller, digested units are taken up by the cells lining the intestines into the bloodstream. Assimilation goes a step further, involving the incorporation of digested nutrients into the body's tissues and metabolic pathways. Transport generally refers to the movement of absorbed nutrients throughout the circulatory system. Thus, digestion is the foundational step that precedes absorption and assimilation, making it the correct answer in this context.