This is not a characteristic feature of anatomy of dicotyledonous root

The correct option is "Vascular bundles 15-20".

To understand why this is not a characteristic feature of the anatomy of a dicotyledonous (dicot) root, let us analyze the anatomical characteristics of a dicot root step-by-step:
1. Number of Vascular Bundles: Dicotyledonous roots typically exhibit a limited number of xylem and phloem groups (vascular bundles), usually ranging from two to six (diarch to hexarch conditions). A high number of vascular bundles, such as 15-20 (polyarch condition), is a characteristic feature of monocotyledonous roots, not dicotyledonous roots. Therefore, "Vascular bundles 15-20" is incorrect for a dicot root.
2. Pith: In dicot roots, the central region or pith is very small, undeveloped, or completely absent. In contrast, monocot roots have a large and well-developed pith.
3. Secondary Growth: Dicot roots undergo secondary growth (increase in thickness) due to the activity of the vascular cambium and cork cambium, which is absent in monocot roots.
4. Radial Vascular Bundles: In dicot roots (as well as monocot roots), the xylem and phloem are arranged alternately on different radii, which is known as a radial arrangement.

Thus, since dicot roots have only 2 to 6 vascular bundles, the presence of 15-20 vascular bundles is not a characteristic of dicotyledonous root anatomy.