Why D? And why not the others?

Correct answer: D. The small intestine

Despite its modest name, the small intestine is by far the longest stretch of the digestive tract, running about 6.7 to 7.6 meters, four to five times the length of the large intestine's roughly 1.5 meters. The names refer to diameter, not length: the large intestine is simply wider. All that length exists because the small intestine does the heavy lifting of nutrient absorption, and its lining of folds and finger-like villi multiplies the absorptive surface enormously. The large intestine is the natural trap purely because of its name. The esophagus is only about 25 centimeters of transport tube, and the stomach is a pouch rather than a long passage.

Source: Encyclopaedia Britannica: small intestine; large intestine