Secondary xylem and phloem in dicot stem are produced by Phellogen, Apical meristems, Axillary meristems, or Vascular cambium?

Secondary xylem and phloem in dicot stem are produced by vascular cambium. Vascular cambium is present between the primary xylem and phloem. Secondary xylem grows towards inside of the cambium and secondary phloem grows towards outside of the cambium.Thus, the correct answer is option D.