In Opuntia, the stem is phylloclade. Let's understand what a phylloclade is and why it is the correct answer for this question.

Phylloclade:
A phylloclade is a modified stem that performs the functions of a leaf. It is a flattened, green, photosynthetic structure that resembles a leaf in appearance and function. Phylloclades have the ability to carry out photosynthesis, just like leaves, and help in the production of food for the plant.

Explanation:
Opuntia belongs to the family Cactaceae, and it is commonly known as the prickly pear cactus. The stem of Opuntia is modified into a phylloclade, which is an adaptation to withstand arid conditions. The transformation of the stem into a phylloclade helps the plant to conserve water and reduce transpiration.

Opuntia plants have reduced or absent leaves, and the stem takes up the role of photosynthesis. The stem is flattened and fleshy, with spines and glochids (tiny barbed hairs) for protection against herbivores. The phylloclades of Opuntia are green and succulent, allowing the plant to carry out photosynthesis efficiently.

The presence of phylloclades in Opuntia is an example of convergent evolution, where plants from different lineages independently evolve similar adaptations to survive in arid environments. Other examples of plants with phylloclades include some species of Euphorbia, Ruscus, and Asparagus.

In conclusion, the stem of Opuntia is modified into a phylloclade, which resembles a leaf in appearance and functions as the primary site for photosynthesis. This adaptation allows Opuntia to thrive in arid conditions by reducing water loss through transpiration.