Explanation:

The alimentary canal, or digestive tract, is a long, continuous tube that runs from the mouth to the anus. Among its various parts, the small intestine is by far the longest. In adult humans, it measures approximately $6$ to $7$ meters in length, providing a vast surface area for nutrient absorption. Option (C) is correct because the small intestine's extensive length is crucial for its primary role in digestion and absorption. Option (A) is incorrect; the oesophagus (food pipe) is relatively short, about $25$ $\text{cm}$. Option (B) is incorrect; the stomach is a muscular, J-shaped organ, typically about $25$ $\text{cm}$ long. Option (D) is incorrect; the large intestine is wider but significantly shorter than the small intestine, measuring about $1.5$ meters.