Anatomically, a fairly old dicotyledonous root can be distinguished from a dicotyledonous stem by the position of the protoxylem. The correct answer is option 'C'.

Explanation:
Dicotyledonous roots and stems have distinct anatomical features that allow for their differentiation. The position of the protoxylem is one such characteristic that can be used to distinguish between the two.

1. Dicotyledonous Root Anatomy:
A dicotyledonous root typically consists of several tissue layers, including the epidermis, cortex, endodermis, pericycle, vascular tissue (xylem and phloem), and pith (central region).

- Epidermis: The outermost layer of cells that covers the root surface.
- Cortex: The layer of cells located beneath the epidermis, which functions in storage and transport of nutrients.
- Endodermis: A single layer of cells that surrounds the vascular tissue and regulates water and nutrient uptake.
- Pericycle: The outermost layer of the vascular cylinder, which gives rise to lateral roots.
- Vascular Tissue: The xylem, responsible for water and mineral transport, is located towards the center, while the phloem, responsible for sugar transport, is located towards the periphery.
- Pith: The central region of the root, consisting of parenchyma cells.

2. Dicotyledonous Stem Anatomy:
A dicotyledonous stem also consists of several tissue layers, including the epidermis, cortex, endodermis, pericycle, vascular tissue (xylem and phloem), and pith (central region).

- Epidermis: The outermost layer of cells that covers the stem surface.
- Cortex: The layer of cells located beneath the epidermis, which functions in storage and support.
- Endodermis: A single layer of cells that surrounds the vascular tissue and regulates water and nutrient transport.
- Pericycle: The outermost layer of the vascular cylinder, which gives rise to lateral branches.
- Vascular Tissue: The xylem, responsible for water and mineral transport, is located towards the center, while the phloem, responsible for sugar transport, is located towards the periphery.
- Pith: The central region of the stem, consisting of parenchyma cells.

3. Position of Protoxylem:
The protoxylem refers to the first-formed xylem elements during root or stem development. In dicotyledonous roots, the protoxylem is located towards the center of the vascular cylinder, closer to the pith. In contrast, in dicotyledonous stems, the protoxylem is located towards the periphery of the vascular bundle, closer to the phloem.

By observing the position of the protoxylem, one can differentiate between a fairly old dicotyledonous root and a dicotyledonous stem. If the protoxylem is found towards the center of the vascular bundle, it indicates a root, while if it is found towards the periphery, it indicates a stem. Therefore, the correct answer for distinguishing between the two is option 'C' - the position of the protoxylem.