The end product of fat digestion is fatty acid. When we eat fats, they are broken down into smaller molecules through a process called digestion. Digestion of fats mainly occurs in the small intestine, where enzymes called lipases break down fats into fatty acids and glycerol. Fatty acids are then absorbed into the bloodstream and transported to the liver, where they are either used for energy or stored as fat in adipose tissue. Therefore, the correct answer is "fatty acid".