What is the main function of the stomach?

The primary function of the stomach is to digest food and convert it into a semi-liquid substance known as chyme. The stomach achieves this through a combination of mechanical and chemical processes. Mechanically, the stomach muscles contract to churn the food, mixing it with gastric juices. These gastric juices contain hydrochloric acid and digestive enzymes, which break down the food into simpler components. This process prepares the food for further digestion and absorption in the small intestine.

In contrast, other functions listed are not carried out by the stomach. For instance, the absorption of nutrients primarily occurs in the small intestine, which is where most nutrients are absorbed into the bloodstream. The excretion of waste is managed by the intestines and the urinary system rather than the stomach. Additionally, bile production occurs in the liver, where it is then stored in the gallbladder, ready to aid in the digestion of fats in the small intestine, rather than being a function of the stomach itself. Therefore, the correct choice emphasizes the stomach's key role in food digestion and the transformation of food into chyme before it moves on to the intestines.