The complete digestion of carbohydrates, proteins, and fats occurs in the small intestine.

Option 1: Stomach - While the stomach plays a role in digestion by breaking down proteins, it does not fully digest carbohydrates or fats. Carbohydrates begin digestion in the mouth, and fats are not broken down until they reach the small intestine.

Option 2: Liver - The liver does not directly participate in the digestion process. However, it produces bile, which is stored in the gallbladder and released into the small intestine to aid in the digestion of fats.

Option 3: Small intestine - The small intestine is where the complete digestion and absorption of carbohydrates, proteins, and fats occur. It is lined with specialized cells that produce enzymes to break down these macronutrients into smaller molecules that can be absorbed by the body.

Option 4: Large intestine - The large intestine primarily absorbs water and electrolytes from the remaining undigested material after digestion has occurred in the small intestine. It does not play a significant role in the digestion of carbohydrates, proteins, or fats.

In conclusion, option 3 (small intestine) is the correct answer as it is where the complete digestion of carbohydrates, proteins, and fats takes place.