The correct answer is D. Just below the root tip.

The radicle (primary root) of a bean seedling grows most rapidly in the region just below the root tip, also known as the zone of elongation. This region is responsible for the rapid growth of the root, as cells in this area are actively dividing and expanding.

Here's a breakdown of the root tip regions:

- Root tip (apex): Cells are actively dividing (zone of cell division).

- Just below the root tip (zone of elongation): Cells are expanding and growing, leading to rapid root growth.

- Further back (zone of maturation): Cells have finished growing and are now mature and functional.

The other options are not correct:

A. The root tip is the region of active cell division, but not the most rapid growth.

B. Just above the root tip is not a specific region associated with rapid growth.

C. Just around the root tip is a vague term and not a specific region associated with rapid growth.