The correct option is C Stem
Bougainvillaea is the modified thorn formed from the stem. In plant morphology, thorns, spines, prickles are all hard structures with sharp ends. They generally help protect the plant from the animals. Thorns are modified branches or stems. Thorns have vascular bundles inside. They control transpiration by reducing the vegetative growth. In Bougainvillea, the axillary bud develops into thorns.
Thus, the correct answer is option A.