Concept:Secondary growth in plants results from the activity of lateral meristems that add new tissues to stems and roots.Explanation:Secondary growth increases the thickness or girth of plant organs. Two lateral meristems are responsible for this process. The vascular cambium produces secondary xylem and secondary phloem. The cork cambium, also called phellogen, produces the periderm, which includes cork and phelloderm. Phelloderm is a product of phellogen, not a meristem itself, so it cannot bring about secondary growth. Procambium is a primary meristem, not a lateral meristem. Therefore, the correct combination of meristems is vascular cambium and phellogen. Together, they cause the increase in diameter of stems and roots.Answer:D. vascular cambium and phellogen