Answer: A. Typhlosole in earthworm
Explanation:
The innermost layer lining the lumen of the alimentary canal is the mucosa. This layer forms irregular folds (rugae) in the stomach and small finger-like foldings called villi in the small intestine. The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance. These modifications increase the surface area enormously.
In the alimentary canal of earthworm, the characteristic feature of the intestine after 26th segment except the last 23rd-25th segments is the presence of internal median fold of dorsal wall called typhlosole. This increases the effective area of absorption in the intestine.
Reference: NCERT Biology Text, Class XI, Unit II: Structural Organisation in Plants and Animals, Chapter 7: Structural Organisation in Animals, Page no: 108 & NCERT Biology Text, Class: XI, Unit: Human Physiology, Chapter: Digestion and Absorption, Page no: 260.