The correct answer is option B - Cambium.

Explanation:
The girth or thickness of the stem or root increases due to the presence of a specialized tissue called cambium. Cambium is a thin layer of meristematic tissue located between the xylem and phloem in the vascular bundles of plants.

1. What is cambium?
Cambium is a type of lateral meristem that is responsible for the secondary growth of plants. It is a thin layer of actively dividing cells that are capable of producing new cells. Cambium cells divide and differentiate into two types of tissues: xylem and phloem. The cambium that produces secondary xylem is called the vascular cambium, while the cambium that produces secondary phloem is called the cork cambium.

2. Role of cambium in increasing girth:
The activity of the vascular cambium leads to the formation of secondary xylem cells towards the inside and secondary phloem cells towards the outside. As these cells are produced, they add to the thickness of the stem or root, resulting in an increase in girth. This process is known as secondary growth.

3. Vascular cambium and secondary xylem:
The vascular cambium produces secondary xylem cells towards the center of the stem or root. These cells are responsible for the conduction of water and minerals from the roots to the rest of the plant. As new secondary xylem cells are added, the stem or root becomes thicker.

4. Vascular cambium and secondary phloem:
The vascular cambium also produces secondary phloem cells towards the outer side of the stem or root. These cells are responsible for the transport of organic nutrients throughout the plant. The addition of new secondary phloem cells further contributes to the increase in girth.

In conclusion, the girth or thickness of the stem or root increases due to the activity of the cambium, which produces new secondary xylem and secondary phloem cells. This process is known as secondary growth and is responsible for the increase in girth of the stem or root.