The correct answer is option 'C' - Intrafascicular cambium.

Intrafascicular cambium is a meristematic region present between the xylem and phloem in open vascular bundles. To understand this in detail, let's break down the components and functions of a vascular bundle.

1. Vascular Bundles:
Vascular bundles are structures found in the stems, roots, and leaves of plants. They consist of various tissues that transport water, nutrients, and sugars throughout the plant. Vascular bundles are of two types: open and closed.

2. Open Vascular Bundles:
Open vascular bundles are found in dicotyledonous plants. They consist of the following components:

- Xylem: Xylem is responsible for transporting water and minerals from the roots to the other parts of the plant. It is composed of tracheids and vessel elements.
- Phloem: Phloem is responsible for transporting organic molecules, such as sugars, from the leaves to other parts of the plant. It consists of sieve tube elements and companion cells.

3. Meristematic Region:
Meristematic tissues are undifferentiated cells that have the ability to divide and differentiate into various specialized cells. They are responsible for plant growth and development. Meristems are classified into three types: apical meristem, lateral meristem, and intercalary meristem.

4. Intrafascicular Cambium:
Intrafascicular cambium is a type of lateral meristem present between the xylem and phloem in open vascular bundles. It is derived from the procambium, which is the primary meristem responsible for the formation of primary vascular tissues. The intrafascicular cambium gives rise to secondary xylem (towards the inside) and secondary phloem (towards the outside).

The presence of the intrafascicular cambium allows the vascular bundle to increase in girth through the production of secondary xylem and phloem. This secondary growth contributes to the thickening of stems and roots, providing mechanical support to the plant.

In summary, the meristematic region present between the xylem and phloem in open vascular bundles is called the intrafascicular cambium. It plays a crucial role in secondary growth by producing secondary xylem and secondary phloem.