Which organ of the human gastrointestinal system is responsible for synthesizing bile juice?

The correct option is Liver.

Detailed Explanation:

1. Liver: The liver is the largest gland in the human body. One of its major functions in the digestive system is the synthesis and secretion of bile juice. Bile contains bile salts that emulsify fats, breaking down large fat globules into smaller droplets to increase the surface area for enzyme action.

2. Gallbladder: The gallbladder does not produce bile; rather, it serves as a storage organ that stores and concentrates the bile produced by the liver until it is released into the small intestine.

3. Stomach: The stomach secretes gastric juices containing hydrochloric acid (HCl) and pepsin, which primarily aid in breaking down proteins.

4. Pancreas: The pancreas produces pancreatic juice containing digestive enzymes such as trypsin, lipase, and pancreatic amylase, as well as bicarbonate to neutralize stomach acid.

Thus, the organ responsible for producing bile juice is the Liver.