The correct answer is option 1: the stomach contains acid. The stomach produces hydrochloric acid, which helps in the digestion of food. It breaks down proteins and kills bacteria that may be present in the food we consume. The acid in the stomach also helps in initiating the process of breaking down carbohydrates and fats.

Option 2, the small intestine, does not contain acid. Instead, it receives partially digested food from the stomach and continues the digestion process using enzymes produced in the pancreas and small intestine itself.

Option 3, the appendix, is a small pouch attached to the large intestine and does not play a significant role in digestion.

Option 4, the colon, is part of the large intestine and is responsible for absorbing water and electrolytes from the leftover waste, but it does not contain acid.