Hello again, Bakere!
a) Mitosis, the dividing of a single cell into two daughter cells, is a way for the plant to stay healthy-looking and strong. All cells die eventually, and they must be replaced by new, fresh ones. This replacement occurs through mitosis. Mitosis also allows the plant to grow, as mitosis is found more frequently in growing areas than ones that are finished growing. Since mitosis makes more cells, you will find much more mitosis occurring in a new shoot than in an old tree, because the old tree doesn't need a bunch of new cells to continue growing.
b) Mitosis is more frequent on the outside of the root because new cells are needed constantly as multitudes of them are scrubbed away by rough dirt. On the inside of the root, however, there is little to no exposure to harsh soil erosion. Because of this, mitosis is needed less here - only to replace the dying cells.
c) The order for the letters should be: A, E, D, B, C. I'm including a picture to help you understand why. :)
Hope this helps!!