The duodenum protects the small intestines from chime by secreting:

The duodenum plays a crucial role in the digestive system by acting as the first section of the small intestine, where it receives chyme from the stomach. To protect the lining of the small intestine from the acidic chyme, the duodenum secretes sodium bicarbonate. This bicarbonate neutralizes the acid, ensuring that the environment in the small intestine is suitable for enzyme activity and nutrient absorption.

Mucous does provide some protection to the gastrointestinal tract by lubricating its surfaces and forming a barrier against digestive acids and enzymes. However, in the context of the duodenum specifically protecting the small intestine from the acidic chyme, sodium bicarbonate is the key substance involved.

The other substances mentioned, such as bile and pepsin, have different roles. Bile is produced in the liver and aids in the emulsification and absorption of fats, while pepsin is an enzyme involved in protein digestion, active primarily in the stomach rather than in the duodenum. Thus, sodium bicarbonate is the correct answer as it directly assists the duodenum in neutralizing the acidic chyme to protect the small intestines.