In Opuntia, spines are modification of
Plants bear modified leaves to perform some special functions besides photosynthesis and transpiration. The main reason for modification is in response to certain environmental conditions. In Opuntia, the stem is modified into a green flattened structure called as phylloclade which has on its surface a number of spines. These spines are the modified leaves which not only helps check transpiration but also protects the plants from herbivorous animals.
Thus, the correct answer is option C.