A cactus belongs to the Kingdom plantae because
they are photosynthetic
they are multicellular
All eukaryotic, multicellular, photosynthetic organisms are classified under the kingdom plantae. All the flowering and non flowering plants, ferns and mosses are included in the Kingdom Plantae. Cactus is a photosynthetic, multicellular eukaryotic organism. So, it belongs to the kingdom Plantae.