Answer:
(c) Vascular Cambium
Step-by-step explanation:
The primary source of stem thickening in plants is the vascular cambium. The vascular cambium is the type of cell involved in secondary growth of a plant, which is when the plant grows wider. This type of cell is found in the stems of plants, and it divides and creates new cells, causing the stem to grow thicker as new cells are added. You can see recorded evidence of this type of growth when you look at the rings of trees. Each ring represents a growing season, where the tree added girth as well as height. Thicker rings indicate better growing seasons because the vascular cambium had access to enough energy to divide more frequently.