In the human digestive system, the process of digestion starts in the buccal cavity, which is more commonly known as the mouth. The mouth plays a crucial role in the digestion process as it is responsible for breaking down the food into smaller pieces through the process of mastication or chewing.

Option 1, the esophagus, is incorrect as the esophagus is a muscular tube that connects the mouth to the stomach. It is not involved in the initial process of digestion.

Option 3, the duodenum, is also incorrect as the duodenum is the first part of the small intestine where further digestion and absorption of nutrients occur, but it does not initiate the process of digestion.

Option 4, the stomach, is partially correct as the stomach is involved in the digestion process, but it is not where digestion starts. The stomach further breaks down the food and mixes it with digestive enzymes and acids.

In summary, the correct answer is option 2, the buccal cavity or the mouth, where the process of digestion begins with chewing and saliva mixing with the food.