The correct answer is option 2: Small intestine. The small intestine is the longest part of the human alimentary canal, which is the system responsible for digesting and absorbing nutrients from food.

To understand why the small intestine can be of maximum length, let`s briefly look at the other options:

1. Stomach: While the stomach is an important organ for digestion, it is not the longest part of the alimentary canal. It primarily serves to store and digest food.

2. Small intestine: The small intestine is divided into three sections, namely the duodenum, jejunum, and ileum. It is responsible for the majority of nutrient absorption. The inner lining of the small intestine contains numerous tiny, finger-like projections called villi, which increase the surface area for absorption, leading to its longer length.

3. Large intestine: The large intestine, also known as the colon, follows the small intestine in the digestive process. Its primary function is to absorb water and electrolytes from undigested food, consolidate waste material, and eliminate it from the body. However, it is shorter in length compared to the small intestine.

4. Rectum: The rectum is the final part of the colon. It stores feces before they are eliminated