Strolling in the sonoran desert, you come upon a stately saguaro cactus. the large central "trunks" of this cactus are covered with numerous smaller spines. the central stalks are modified used for water storage and spines are modified used for protection.

I believe  that the central stalks are modified stems used for water storage and spines are modified leaves used for protection. Desert environment is normally hot and warm and therefore plants and animals have to have a mechanism to conserve water, the plants are very different such that they are often swollen, spiny, and have tiny leaves that are rarely bright green. These are as a results of adaptations to challenges of the desert.

The answer is d i think