The multicellular organism containing cells that have a nucleus and a cell wall as well as the ability to conduct photosynthesis would most likely fit into the domain Eukarya.
Eukarya is one of the three domains of life, and includes all organisms that have eukaryotic cells, which are characterized by the presence of a nucleus and other membrane-bound organelles. This domain includes a wide range of organisms, including animals, plants, fungi, and protists. The presence of a cell wall and the ability to conduct photosynthesis would suggest that this organism is a plant or algae, both of which fall under the domain Eukarya.