The correct answer is option 'C' which is the stem of a cactus plant.

Stem as the primary site of photosynthesis:
The stem of a cactus plant performs photosynthesis. Unlike most other plants, cacti have modified leaves called spines which are not involved in photosynthesis. Instead, the stem of a cactus takes on the role of conducting photosynthesis.

Adaptations of cactus stems for photosynthesis:
Cactus stems have several adaptations that enable them to carry out photosynthesis efficiently:

1. Chlorophyll-containing cells: The stem of a cactus contains chlorophyll, the pigment responsible for capturing sunlight and converting it into energy through photosynthesis. These chlorophyll-containing cells are located in the green outer layer of the stem.

2. Reduced surface area: Cactus stems have a reduced surface area compared to leaves. This adaptation helps to minimize water loss through transpiration, as the surface area available for evaporation is reduced.

3. Thick and succulent tissue: The stem of a cactus is typically thick and succulent, meaning it is fleshy and capable of storing water. This adaptation allows the cactus to survive in arid environments where water is scarce. The stored water can be used for photosynthesis during times of drought or when the plant is unable to take up water from the soil.

4. Waxy coating: The stem of a cactus is often covered with a waxy coating, known as a cuticle. This cuticle helps to reduce water loss by preventing excessive evaporation from the stem surface.

Conclusion:
In conclusion, the stem of a cactus plant performs photosynthesis. It has adapted to carry out this process efficiently by containing chlorophyll, reducing surface area, having thick and succulent tissue, and possessing a waxy coating. These adaptations allow the cactus to survive in arid environments with limited water availability.