The answer is option 'B': Vascular cambium.

Explanation:
The narrow layer of thin-walled cells found between the phloem/bark and wood of a dicot is known as the vascular cambium. It is a meristematic tissue that is responsible for the secondary growth of the plant. Secondary growth refers to the increase in girth or thickness of the plant.

Function of the Vascular Cambium:
The vascular cambium is responsible for the production of secondary xylem (wood) towards the inner side and secondary phloem (bark) towards the outer side of the stem or root. It adds new layers of xylem and phloem, which contribute to the radial growth of the plant.

Structure of the Vascular Cambium:
The vascular cambium is a thin layer of cells that is located between the primary xylem and primary phloem. It consists of actively dividing cells called cambial initials. These initials divide repeatedly, with one cell differentiating into xylem and the other into phloem. The newly formed cells contribute to the secondary growth of the plant.

Significance of the Vascular Cambium:
The vascular cambium plays a crucial role in the development of woody stems and roots. It is responsible for the formation of annual rings in tree trunks, which can be used to determine the age of a tree. The secondary xylem produced by the vascular cambium provides mechanical support to the plant and conducts water and minerals from the roots to the leaves. The secondary phloem transports organic nutrients from the leaves to other parts of the plant.

In conclusion, the vascular cambium is a narrow layer of thin-walled cells found between the phloem/bark and wood of a dicot. It is responsible for the secondary growth of the plant and the production of secondary xylem and phloem.