**Transpiration Organ in Plants:**
The correct answer to the question is option 'A' - Epidermis.

**Explanation:**
The transpiration organ in plants is the epidermis. The epidermis is the outermost layer of cells covering the leaves, stems, and roots of plants. It serves as a protective barrier against mechanical injury, pathogens, and excessive water loss. It also plays a crucial role in the process of transpiration.

**Transpiration:**
Transpiration is the process by which plants lose water in the form of water vapor through the stomata, which are tiny openings present on the epidermis of leaves and stems. It is an essential process for plants as it helps in the absorption and transport of water and nutrients, cooling the plant, and maintaining turgidity.

**Role of Epidermis in Transpiration:**
The epidermis of leaves and stems contains specialized cells called guard cells, which surround and control the opening and closing of the stomata. When the plant needs to release excess water vapor, the guard cells open the stomata, allowing water to evaporate from the leaf surface. This process is known as transpiration.

**Structure of Epidermis:**
The epidermis is made up of tightly packed cells with a waxy layer called the cuticle on the outer surface. The cuticle helps in reducing water loss by acting as a waterproof barrier. However, the majority of transpiration occurs through the stomata.

**Importance of Transpiration:**
Transpiration is essential for several reasons:
1. Water and nutrient absorption: Transpiration creates a continuous flow of water and nutrients from the roots to the leaves, allowing plants to receive the necessary resources for growth and development.
2. Cooling effect: Transpiration helps in cooling the plant by releasing water vapor, similar to how sweating cools our bodies.
3. Maintaining turgidity: Transpiration maintains turgidity in plant cells, preventing wilting and maintaining the overall health and structure of the plant.

In conclusion, the epidermis of plants serves as the transpiration organ because it contains the stomata, which are responsible for the release of water vapor during the process of transpiration.