In Opuntia, the stem is modified into a flattened green structure of perform the function of leaves, (i.e., photosynthesis). Cite some other example of modifications of plant parts for the purpose of photosynthesis.

Explanation

Opunita is a xerophytic plant, in which leaves are modified into spine to reduce the rate of transpiration and they donot perform the photosynthesis at all.
So function of photosynthesis in Opuntia plant is performed by stem which is thick fleshy and flattened structure containing chlorophyll and stores food. It is known as phylloclade.
Sometimes , the stem, i.e., about one internode long modifies into a leaf like structure to carry out photosynthesis , as in Asparagus. They are present in axil of scale leaves while true leaves aer reduced to scales or spines.
Similarly in some plants, roots become assimilatory as in the case of Trapa and Tinospora. These roots grow outside the soil, develop chlorophyll in them and perform photosynthesis.