Final answer:
In cacti, photosynthesis occurs in the stem.
Step-by-step explanation:
Photosynthesis in plants primarily occurs in leaves. However, cacti are an exception because they have adapted to dry environments. In cacti, the stem is responsible for photosynthesis. The stem of a cactus contains specialized tissue called the parenchyma, which performs photosynthesis. The stem is capable of carrying out this process due to its ability to store water and produce chlorophyll.