The correct answer is C. Cambium.
The cambium is a lateral meristem that gives rise to secondary xylem and phloem. It is a thin layer of cells that lies between the xylem and phloem of a stem or root. The cambium is responsible for the growth in thickness of a plant.
Phloem is a vascular tissue that transports food from the leaves to other parts of the plant. It is made up of sieve tubes, companion cells, and phloem parenchyma cells.
Xylem is a vascular tissue that transports water and minerals from the roots to the leaves. It is made up of tracheids, vessels, and xylem parenchyma cells.
Medullary ray is a parenchyma tissue that lies in the center of a stem or root. It is responsible for storage and transport of nutrients.
Here is a diagram of a cross-section of a stem, showing the location of the cambium:
The cambium is the thin layer of cells between the xylem and phloem. It is responsible for the growth in thickness of the stem.