Explanation:

In cacti, the stem is modified to perform photosynthesis and store water, making it thick, green, and fleshy.

• The leaves are reduced to spines to minimize water loss.
• Roots help in absorbing water but are not the primary modified part.
• Flowers are reproductive structures and are not modified for survival in deserts.

Thus, the cactus is a modified stem adapted for water storage and photosynthesis.