**Explanation:**

**Transportation of Auxin in Plants:**
Auxin is a plant hormone that plays a crucial role in various plant growth and developmental processes. It is primarily synthesized in the apical meristem (shoot tip) and transported to different parts of the plant. The main mode of auxin transport is polar, which means it occurs in a specific direction.

**Transport Direction:**
The correct answer is option 'A': From the shoot tip in the downward direction. Auxin is transported from the shoot tip towards the lower parts of the plant, including the root system. This transport is known as basipetal transport.

**Auxin Transport Mechanism:**
Auxin transport occurs through various pathways within the plant. The primary pathway for auxin transport is the vascular system, which consists of the xylem and phloem.

**Vascular System:**
The vascular system is responsible for the transport of water, minerals, sugars, and hormones throughout the plant. The xylem transports water and minerals from the roots to the shoots, while the phloem transports sugars and hormones in both upward and downward directions.

**Xylem Transport:**
In the xylem, auxin is mainly transported via a process called apoplastic transport. It moves through the cell walls and intercellular spaces. This movement is facilitated by passive diffusion, as auxin is present in higher concentrations in the shoot tip compared to the root tip.

**Phloem Transport:**
In addition to the xylem, auxin can also be transported through the phloem. The phloem transport of auxin occurs in the form of complex molecules called auxin conjugates. These conjugates are synthesized in the shoot tip and transported to the root system through the phloem. Once in the root, they are converted back to active auxin, contributing to the basipetal transport.

**Special Transport System in the Root:**
Option 'D', which states that auxin is transported by a special transport system in the root, is incorrect. While the root does play a role in auxin transport, it does not have a specialized transport system solely dedicated to auxin transport. The transport of auxin in the root occurs primarily through the vascular system, similar to the shoot.

In conclusion, native auxin is primarily transported from the shoot tip in the downward direction through the vascular system, including the xylem and phloem. This transport is essential for the regulation of various plant growth processes, such as tropisms, root development, and apical dominance.