What substance does the liver primarily produce?

The liver primarily produces bile, which is essential for digestion, particularly the emulsification and absorption of fats in the small intestine. Bile contains bile salts, which help break down fats into smaller droplets, making them more accessible for digestive enzymes. It also aids in the elimination of waste products, particularly bilirubin, a byproduct of red blood cell breakdown.

While the liver is involved in the metabolism of insulin and glucagon—hormones produced by the pancreas that regulate blood sugar levels—it does not produce these hormones itself. Additionally, although the liver does play a role in urea production as part of the urea cycle to remove ammonia from the body, its primary function in this context is not the direct production of urea but rather the regulation of nitrogen metabolism.

Thus, the production of bile is a key function of the liver that supports important digestive processes, making it the correct answer.