Final answer:
The storage of food, mixing food, and the secretion of acidic gastric juices occur in the stomach.
Step-by-step explanation:
The storage of food, mixing food, and the secretion of acidic gastric juices occur in the stomach. The stomach is a sac-like organ that secretes gastric digestive juices and has a highly acidic environment, with a pH between 1.5 and 2.5. This acidity is necessary for the chemical breakdown of food and the extraction of nutrients. It can expand to up to 20 times its resting size to accommodate food.