Answer: The correct answer for the blank is -
A. is part of a plant's secondary growth.
Step-by-step explanation:
Vascular cambium can be described as a thin layer of unspecialized meristem cells that is present in woody plants and separates two other types of plant vascular tissue, which are phloem and xylem.
These cells multiply and divide with the secondary growth of plant in which it increase the girth size or width of plant.
It is the source of both secondary phloem growth outside to the bark and secondary xylem growth inwards that is towards the pith.
Thus, option A) is the right answer.