Final answer:
An organism with cells containing a nucleus that is multicellular and photosynthetic is classified under Kingdom Plantae, which is distinct from the heterotrophic Kingdom Animalia where organisms ingest food.
Step-by-step explanation:
An organism that has a nucleus in its cells, is multicellular, and produces its own food through photosynthesis belongs to the Kingdom Plantae. This classification is based on the presence of a nucleus, which signifies that the organism is eukaryotic, and its ability to perform photosynthesis, which indicates that it is autotrophic. Unlike organisms in Kingdom Animalia, which are heterotrophic and ingest their food, plants can generate their own energy through the conversion of sunlight. This ability to photosynthesize is a defining characteristic of the plant kingdom and separates it from animals, fungi, and other life forms.
The process of classifying organisms into kingdoms is a part of taxonomy, a branch of biology that deals with the grouping and naming of organisms. Scholars use taxonomy to categorize life into domains and kingdoms, with humans and other animals included in Kingdom Animalia due to their heterotrophic nature and reliance on ingestion of organic material for energy.