Hydrochloric acid is produced in the stomach. The stomach is a vital organ in the digestive system responsible for breaking down food and aiding in digestion. It secretes several gastric juices, including hydrochloric acid.

Option 2, the liver, is not the correct answer. While the liver does play a crucial role in digestion by producing bile to break down fats, it does not produce hydrochloric acid.

Option 3, the pancreas, is also not the correct answer. The pancreas does secrete digestive enzymes, but it does not produce hydrochloric acid. Instead, it releases bicarbonate ions to neutralize stomach acid in the intestines.

Option 4, the small intestine, is not the correct answer either. While the small intestine is responsible for further digestion and absorption of nutrients, it does not produce hydrochloric acid.

Therefore, the correct answer is option 1, the stomach, where hydrochloric acid is produced to aid in the digestion process.