Final answer:
The organism described is best classified under the kingdom Plantae as it is multicellular, has a nucleus, stays still, and photosynthesizes when placed in light.
Step-by-step explanation:
The best kingdom that describes the described organism is Plantae. Kingdom Plantae includes multicellular, autotrophic organisms that use photosynthesis to meet their energy demands. This kingdom is characterized by the presence of a nucleus and all the cells in the organism being the same in their function.