In the cauliflower plant, the useful part is the young inflorescence. This refers to the compact cluster of flowers that forms the cauliflower head. The cauliflower plant belongs to the Brassica oleracea species, and the thickened, undeveloped flower buds are what we commonly consume as cauliflower. These flower buds are harvested before they fully open and are considered the useful part of the plant. The underground stem and root are not the edible parts of the cauliflower plant. While leaves are present in the plant, they are not typically consumed. Therefore, option 3 - young inflorescence, is the correct answer.